CSS Syntax Module Level 3

1. Introduction

This section is not normative.

This module defines the abstract syntax and parsing of CSS stylesheets and other things which use CSS syntax (such as the HTML style attribute).

It defines algorithms for converting a stream of Unicode code points (in other words, text) into a stream of CSS tokens, and then further into CSS objects such as stylesheets, rules, and declarations.

1.1. Module interactions

This module defines the syntax and parsing of CSS stylesheets. It supersedes the lexical scanner and grammar defined in CSS 2.1.

2. Description of CSS’s Syntax

This section is not normative.

A CSS document is a series of style rules—which are qualified rules that apply styles to elements in a document—and at-rules—which define special processing rules or values for the CSS document.

A qualified rule starts with a prelude then has a {}-wrapped block containing a sequence of declarations. The meaning of the prelude varies based on the context that the rule appears in—for style rules, it’s a selector which specifies what elements the declarations will apply to. Each declaration has a name, followed by a colon and the declaration value. Declarations are separated by semicolons.

p > a {
  color: blue;
  text-decoration: underline;
}

At-rules are all different, but they have a basic structure in common. They start with an "@" code point followed by their name as a CSS keyword. Some at-rules are simple statements, with their name followed by more CSS values to specify their behavior, and finally ended by a semicolon. Others are blocks; they can have CSS values following their name, but they end with a {}-wrapped block, similar to a qualified rule. Even the contents of these blocks are specific to the given at-rule: sometimes they contain a sequence of declarations, like a qualified rule; other times, they may contain additional blocks, or at-rules, or other structures altogether.

@import "my-styles.css";

@page :left {
  margin-left: 4cm;
  margin-right: 3cm;
}

@media print {
  body { font-size: 10pt }
}

Property names and at-rule names are always ident sequences, which have to start with an ident-start code point, two hyphens, or a hyphen followed by an ident-start code point, and then can contain zero or more ident code points. You can include any code point at all, even ones that CSS uses in its syntax, by escaping it.

The syntax of selectors is defined in the Selectors spec. Similarly, the syntax of the wide variety of CSS values is defined in the Values & Units spec. The special syntaxes of individual at-rules can be found in the specs that define them.

2.1. Escaping

This section is not normative.

Any Unicode code point can be included in an ident sequence or quoted string by escaping it. CSS escape sequences start with a backslash (\), and continue with:

• Any Unicode code point that is not a hex digits or a newline. The escape sequence is replaced by that code point.
• Or one to six hex digits, followed by an optional whitespace. The escape sequence is replaced by the Unicode code point whose value is given by the hexadecimal digits. This optional whitespace allow hexadecimal escape sequences to be followed by "real" hex digits.

  An ident sequence with the value "&B" could be written as \26 B or \000026B.

  A "real" space after the escape sequence must be doubled.

2.2. Error Handling

This section is not normative.

When errors occur in CSS, the parser attempts to recover gracefully, throwing away only the minimum amount of content before returning to parsing as normal. This is because errors aren’t always mistakes—new syntax looks like an error to an old parser, and it’s useful to be able to add new syntax to the language without worrying about stylesheets that include it being completely broken in older UAs.

The precise error-recovery behavior is detailed in the parser itself, but it’s simple enough that a short description is fairly accurate.

• At the "top level" of a stylesheet, an <at-keyword-token> starts an at-rule. Anything else starts a qualified rule, and is included in the rule’s prelude. This may produce an invalid selector, but that’s not the concern of the CSS parser—at worst, it means the selector will match nothing.
• Once an at-rule starts, nothing is invalid from the parser’s standpoint; it’s all part of the at-rule’s prelude. Encountering a <semicolon-token> ends the at-rule immediately, while encountering an opening curly-brace <{-token> starts the at-rule’s body. The at-rule seeks forward, matching blocks (content surrounded by (), {}, or []) until it finds a closing curly-brace <}-token> that isn’t matched by anything else or inside of another block. The contents of the at-rule are then interpreted according to the at-rule’s own grammar.
• Qualified rules work similarly, except that semicolons don’t end them; instead, they are just taken in as part of the rule’s prelude. When the first {} block is found, the contents are always interpreted as a list of declarations.
• When interpreting a list of declarations, unknown syntax at any point causes the parser to throw away whatever declaration it’s currently building, and seek forward until it finds a semicolon (or the end of the block). It then starts fresh, trying to parse a declaration again.
• If the stylesheet ends while any rule, declaration, function, string, etc. are still open, everything is automatically closed. This doesn’t make them invalid, though they may be incomplete and thus thrown away when they are verified against their grammar.

After each construct (declaration, style rule, at-rule) is parsed, the user agent checks it against its expected grammar. If it does not match the grammar, it’s invalid, and gets ignored by the UA, which treats it as if it wasn’t there at all.

3. Tokenizing and Parsing CSS

User agents must use the parsing rules described in this specification to generate the [CSSOM] trees from text/css resources. Together, these rules define what is referred to as the CSS parser.

This specification defines the parsing rules for CSS documents, whether they are syntactically correct or not. Certain points in the parsing algorithm are said to be parse errors. The error handling for parse errors is well-defined: user agents must either act as described below when encountering such problems, or must abort processing at the first error that they encounter for which they do not wish to apply the rules described below.

Conformance checkers must report at least one parse error condition to the user if one or more parse error conditions exist in the document and must not report parse error conditions if none exist in the document. Conformance checkers may report more than one parse error condition if more than one parse error condition exists in the document. Conformance checkers are not required to recover from parse errors, but if they do, they must recover in the same way as user agents.

3.1. Overview of the Parsing Model

The input to the CSS parsing process consists of a stream of Unicode code points, which is passed through a tokenization stage followed by a tree construction stage. The output is a CSSStyleSheet object.

Note: Implementations that do not support scripting do not have to actually create a CSSOM CSSStyleSheet object, but the CSSOM tree in such cases is still used as the model for the rest of the specification.

3.2. The input byte stream

When parsing a stylesheet, the stream of Unicode code points that comprises the input to the tokenization stage might be initially seen by the user agent as a stream of bytes (typically coming over the network or from the local file system). If so, the user agent must decode these bytes into code points according to a particular character encoding.

@charset "utf-8";

Document languages that refer to CSS stylesheets that are decoded from bytes may define an environment encoding for each such stylesheet, which is used as a fallback when other encoding hints are not available or can not be used.

The concept of environment encoding only exists for compatibility with legacy content. New formats and new linking mechanisms should not provide an environment encoding, so the stylesheet defaults to UTF-8 instead in the absence of more explicit information.

Note: [HTML] defines the environment encoding for <link rel=stylesheet>.

Note: [CSSOM] defines the environment encoding for <xml-stylesheet?>.

Note: [CSS-CASCADE-3] defines the environment encoding for @import.

3.3. Preprocessing the input stream

The input stream consists of the filtered code points pushed into it as the input byte stream is decoded.

4. Tokenization

To tokenize a stream of code points into a stream of CSS tokens input, repeatedly consume a token from input until an <EOF-token> is reached, pushing each of the returned tokens into a stream.

Note: Each call to the consume a token algorithm returns a single token, so it can also be used "on-demand" to tokenize a stream of code points during parsing, if so desired.

The output of tokenization step is a stream of zero or more of the following tokens: <ident-token>, <function-token>, <at-keyword-token>, <hash-token>, <string-token>, <bad-string-token>, <url-token>, <bad-url-token>, <delim-token>, <number-token>, <percentage-token>, <dimension-token>, <whitespace-token>, <CDO-token>, <CDC-token>, <colon-token>, <semicolon-token>, <comma-token>, <[-token>, <]-token>, <(-token>, <)-token>, <{-token>, and <}-token>.

• <ident-token>, <function-token>, <at-keyword-token>, <hash-token>, <string-token>, and <url-token> have a value composed of zero or more code points. Additionally, hash tokens have a type flag set to either "id" or "unrestricted". The type flag defaults to "unrestricted" if not otherwise set.
• <delim-token> has a value composed of a single code point.
• <number-token>, <percentage-token>, and <dimension-token> have a numeric value. <number-token> and <dimension-token> additionally have a type flag set to either "integer" or "number". The type flag defaults to "integer" if not otherwise set. <dimension-token> additionally have a unit composed of one or more code points.

Note: The type flag of hash tokens is used in the Selectors syntax [SELECT]. Only hash tokens with the "id" type are valid ID selectors.

4.1. Token Railroad Diagrams

This section is non-normative.

This section presents an informative view of the tokenizer, in the form of railroad diagrams. Railroad diagrams are more compact than an explicit parser, but often easier to read than an regular expression.

These diagrams are informative and incomplete; they describe the grammar of "correct" tokens, but do not describe error-handling at all. They are provided solely to make it easier to get an intuitive grasp of the syntax of each token.

Diagrams with names such as <foo-token> represent tokens. The rest are productions referred to by other diagrams.

comment

newline

whitespace

hex digit

escape

<whitespace-token>

ws*

<ident-token>

<function-token>

<at-keyword-token>

<hash-token>

<string-token>

<url-token>

<number-token>

<dimension-token>

<percentage-token>

<CDO-token>

<CDC-token>

4.2. Definitions

This section defines several terms used during the tokenization phase.

next input code point

The first code point in the input stream that has not yet been consumed.

current input code point

The last code point to have been consumed.

reconsume the current input code point

Push the current input code point back onto the front of the input stream, so that the next time you are instructed to consume the next input code point, it will instead reconsume the current input code point.

EOF code point

A conceptual code point representing the end of the input stream. Whenever the input stream is empty, the next input code point is always an EOF code point.

digit

A code point between U+0030 DIGIT ZERO (0) and U+0039 DIGIT NINE (9) inclusive.

hex digit

A digit, or a code point between U+0041 LATIN CAPITAL LETTER A (A) and U+0046 LATIN CAPITAL LETTER F (F) inclusive, or a code point between U+0061 LATIN SMALL LETTER A (a) and U+0066 LATIN SMALL LETTER F (f) inclusive.

uppercase letter

A code point between U+0041 LATIN CAPITAL LETTER A (A) and U+005A LATIN CAPITAL LETTER Z (Z) inclusive.

lowercase letter

A code point between U+0061 LATIN SMALL LETTER A (a) and U+007A LATIN SMALL LETTER Z (z) inclusive.

letter

An uppercase letter or a lowercase letter.

non-ASCII ident code point

A code point whose value is any of:

• U+00B7

• between U+00C0 and U+00D6

• between U+00D8 and U+00F6

• between U+00F8 and U+037D

• between U+037F and U+1FFF

• U+200C

• U+200D

• U+203F

• U+2040

• between U+2070 and U+218F

• between U+2C00 and U+2FEF

• between U+3001 and U+D7FF

• between U+F900 and U+FDCF

• between U+FDF0 and U+FFFD

• greater than or equal to U+10000

Why these character, specifically?

This matches the list of non-ASCII codepoints allowed to be used in HTML valid custom element names. It excludes a number of characters that appear as whitespace, or that can cause rendering or parsing issues in some tools, such as the direction override codepoints.

Note that this is a weaker set of restrictions than UAX 31 recommends for identifiers (used by languages such as JavaScript to restrict their identifier syntax), allowing things such as starting an identifier with a combining character. Consistency with HTML custom element names (and thus, the ability to write selectors for all custom elements without having to use escapes) was considered valuable, and the set of characters restricted by HTML covers the "high value" restrictions well.

These restrictions do not avoid all possible confusing renderings; mixing characters from LTR and RTL scripts can still result in unexpected visual transposition in most text editors, for example. Source text can contain the restricted characters in non-ident contexts, as well: most of them are completely valid in strings, for example. Even when used in a way that creates invalid CSS, the parsing errors they cause might be limited to something unimportant, while their effect on rendering the source text in code review tools might be significant and/or malicious. For more details on these sorts of "source text attacks", see this Rust-lang blog post (archived).

ident-start code point

A letter, a non-ASCII ident code point, or U+005F LOW LINE (_).

ident code point

An ident-start code point, a digit, or U+002D HYPHEN-MINUS (-).

non-printable code point

A code point between U+0000 NULL and U+0008 BACKSPACE inclusive, or U+000B LINE TABULATION, or a code point between U+000E SHIFT OUT and U+001F INFORMATION SEPARATOR ONE inclusive, or U+007F DELETE.

newline

U+000A LINE FEED. Note that U+000D CARRIAGE RETURN and U+000C FORM FEED are not included in this definition, as they are converted to U+000A LINE FEED during preprocessing.

whitespace

A newline, U+0009 CHARACTER TABULATION, or U+0020 SPACE.

maximum allowed code point

The greatest code point defined by Unicode: U+10FFFF.

ident sequence

A sequence of code points that has the same syntax as an <ident-token>.

Note: The part of an <at-keyword-token> after the "@", the part of a <hash-token> (with the "id" type flag) after the "#", the part of a <function-token> before the "(", and the unit of a <dimension-token> are all ident sequences.

representation

The representation of a token is the subsequence of the input stream consumed by the invocation of the consume a token algorithm that produced it. This is preserved for a few algorithms that rely on subtle details of the input text, which a simple "re-serialization" of the tokens might disturb.

The representation is only consumed by internal algorithms, and never directly exposed, so it’s not actually required to preserve the exact text; equivalent methods, such as associating each token with offsets into the source text, also suffice.

Note: In particular, the representation preserves details such as whether .009 was written as .009 or 9e-3, and whether a character was written literally or as a CSS escape. The former is necessary to properly parse <urange> productions; the latter is basically an accidental leak of the tokenizing abstraction, but allowed because it makes the impl easier to define.

If a token is ever produced by an algorithm directly, rather than thru the tokenization algorithm in this specification, its representation is the empty string.

4.3. Tokenizer Algorithms

The algorithms defined in this section transform a stream of code points into a stream of tokens.

4.3.1. Consume a token

This section describes how to consume a token from a stream of code points. It will return a single token of any type.

Consume comments.

Consume the next input code point.

whitespace

Consume as much whitespace as possible. Return a <whitespace-token>.

U+0022 QUOTATION MARK (")

Consume a string token and return it.

U+0023 NUMBER SIGN (#)

If the next input code point is an ident code point or the next two input code points are a valid escape, then:

1. Create a <hash-token>.
2. If the next 3 input code points would start an ident sequence, set the <hash-token>’s type flag to "id".
3. Consume an ident sequence, and set the <hash-token>’s value to the returned string.
4. Return the <hash-token>.

Otherwise, return a <delim-token> with its value set to the current input code point.

U+0027 APOSTROPHE (')

Consume a string token and return it.

U+0028 LEFT PARENTHESIS (()

Return a <(-token>.

U+0029 RIGHT PARENTHESIS ())

Return a <)-token>.

U+002B PLUS SIGN (+)

If the input stream starts with a number, reconsume the current input code point, consume a numeric token, and return it.

Otherwise, return a <delim-token> with its value set to the current input code point.

U+002C COMMA (,)

Return a <comma-token>.

U+002D HYPHEN-MINUS (-)

If the input stream starts with a number, reconsume the current input code point, consume a numeric token, and return it.

Otherwise, if the next 2 input code points are U+002D HYPHEN-MINUS U+003E GREATER-THAN SIGN (->), consume them and return a <CDC-token>.

Otherwise, if the input stream starts with an ident sequence, reconsume the current input code point, consume an ident-like token, and return it.

Otherwise, return a <delim-token> with its value set to the current input code point.

U+002E FULL STOP (.)

If the input stream starts with a number, reconsume the current input code point, consume a numeric token, and return it.

Otherwise, return a <delim-token> with its value set to the current input code point.

U+003A COLON (:)

Return a <colon-token>.

U+003B SEMICOLON (;)

Return a <semicolon-token>.

U+003C LESS-THAN SIGN (<)

If the next 3 input code points are U+0021 EXCLAMATION MARK U+002D HYPHEN-MINUS U+002D HYPHEN-MINUS (!--), consume them and return a <CDO-token>.

Otherwise, return a <delim-token> with its value set to the current input code point.

U+0040 COMMERCIAL AT (@)

If the next 3 input code points would start an ident sequence, consume an ident sequence, create an <at-keyword-token> with its value set to the returned value, and return it.

Otherwise, return a <delim-token> with its value set to the current input code point.

U+005B LEFT SQUARE BRACKET ([)

Return a <[-token>.

U+005C REVERSE SOLIDUS (\)

If the input stream starts with a valid escape, reconsume the current input code point, consume an ident-like token, and return it.

Otherwise, this is a parse error. Return a <delim-token> with its value set to the current input code point.

U+005D RIGHT SQUARE BRACKET (])

Return a <]-token>.

U+007B LEFT CURLY BRACKET ({)

Return a <{-token>.

U+007D RIGHT CURLY BRACKET (})

Return a <}-token>.

digit

Reconsume the current input code point, consume a numeric token, and return it.

ident-start code point

Reconsume the current input code point, consume an ident-like token, and return it.

EOF

Return an <EOF-token>.

anything else

Return a <delim-token> with its value set to the current input code point.

4.3.2. Consume comments

This section describes how to consume comments from a stream of code points. It returns nothing.

If the next two input code point are U+002F SOLIDUS (/) followed by a U+002A ASTERISK (*), consume them and all following code points up to and including the first U+002A ASTERISK (*) followed by a U+002F SOLIDUS (/), or up to an EOF code point. Return to the start of this step.

If the preceding paragraph ended by consuming an EOF code point, this is a parse error.

Return nothing.

4.3.3. Consume a numeric token

This section describes how to consume a numeric token from a stream of code points. It returns either a <number-token>, <percentage-token>, or <dimension-token>.

Consume a number and let number be the result.

If the next 3 input code points would start an ident sequence, then:

1. Create a <dimension-token> with the same value and type flag as number, and a unit set initially to the empty string.
2. Consume an ident sequence. Set the <dimension-token>’s unit to the returned value.
3. Return the <dimension-token>.

Otherwise, if the next input code point is U+0025 PERCENTAGE SIGN (%), consume it. Create a <percentage-token> with the same value as number, and return it.

Otherwise, create a <number-token> with the same value and type flag as number, and return it.

4.3.4. Consume an ident-like token

This section describes how to consume an ident-like token from a stream of code points. It returns an <ident-token>, <function-token>, <url-token>, or <bad-url-token>.

Consume an ident sequence, and let string be the result.

If string’s value is an ASCII case-insensitive match for "url", and the next input code point is U+0028 LEFT PARENTHESIS ((), consume it. While the next two input code points are whitespace, consume the next input code point. If the next one or two input code points are U+0022 QUOTATION MARK ("), U+0027 APOSTROPHE ('), or whitespace followed by U+0022 QUOTATION MARK (") or U+0027 APOSTROPHE ('), then create a <function-token> with its value set to string and return it. Otherwise, consume a url token, and return it.

Otherwise, if the next input code point is U+0028 LEFT PARENTHESIS ((), consume it. Create a <function-token> with its value set to string and return it.

Otherwise, create an <ident-token> with its value set to string and return it.

4.3.5. Consume a string token

This section describes how to consume a string token from a stream of code points. It returns either a <string-token> or <bad-string-token>.

This algorithm may be called with an ending code point, which denotes the code point that ends the string. If an ending code point is not specified, the current input code point is used.

Initially create a <string-token> with its value set to the empty string.

Repeatedly consume the next input code point from the stream:

ending code point

Return the <string-token>.

EOF

This is a parse error. Return the <string-token>.

newline

This is a parse error. Reconsume the current input code point, create a <bad-string-token>, and return it.

U+005C REVERSE SOLIDUS (\)

If the next input code point is EOF, do nothing.

Otherwise, if the next input code point is a newline, consume it.

Otherwise, (the stream starts with a valid escape) consume an escaped code point and append the returned code point to the <string-token>’s value.

anything else

Append the current input code point to the <string-token>’s value.

4.3.6. Consume a url token

This section describes how to consume a url token from a stream of code points. It returns either a <url-token> or a <bad-url-token>.

Note: This algorithm assumes that the initial "url(" has already been consumed. This algorithm also assumes that it’s being called to consume an "unquoted" value, like url(foo). A quoted value, like url("foo"), is parsed as a <function-token>. Consume an ident-like token automatically handles this distinction; this algorithm shouldn’t be called directly otherwise.

1. Initially create a <url-token> with its value set to the empty string.
2. Consume as much whitespace as possible.
3. Repeatedly consume the next input code point from the stream:

   U+0029 RIGHT PARENTHESIS ())

   Return the <url-token>.

   EOF

   This is a parse error. Return the <url-token>.

   whitespace

   Consume as much whitespace as possible. If the next input code point is U+0029 RIGHT PARENTHESIS ()) or EOF, consume it and return the <url-token> (if EOF was encountered, this is a parse error); otherwise, consume the remnants of a bad url, create a <bad-url-token>, and return it.

   U+0022 QUOTATION MARK (")

   U+0027 APOSTROPHE (')

   U+0028 LEFT PARENTHESIS (()

   non-printable code point

   This is a parse error. Consume the remnants of a bad url, create a <bad-url-token>, and return it.

   U+005C REVERSE SOLIDUS (\)

   If the stream starts with a valid escape, consume an escaped code point and append the returned code point to the <url-token>’s value.

   Otherwise, this is a parse error. Consume the remnants of a bad url, create a <bad-url-token>, and return it.

   anything else

   Append the current input code point to the <url-token>’s value.

4.3.7. Consume an escaped code point

This section describes how to consume an escaped code point. It assumes that the U+005C REVERSE SOLIDUS (\) has already been consumed and that the next input code point has already been verified to be part of a valid escape. It will return a code point.

Consume the next input code point.

hex digit

Consume as many hex digits as possible, but no more than 5. Note that this means 1-6 hex digits have been consumed in total. If the next input code point is whitespace, consume it as well. Interpret the hex digits as a hexadecimal number. If this number is zero, or is for a surrogate, or is greater than the maximum allowed code point, return U+FFFD REPLACEMENT CHARACTER (�). Otherwise, return the code point with that value.

EOF

This is a parse error. Return U+FFFD REPLACEMENT CHARACTER (�).

anything else

Return the current input code point.

4.3.8. Check if two code points are a valid escape

This section describes how to check if two code points are a valid escape. The algorithm described here can be called explicitly with two code points, or can be called with the input stream itself. In the latter case, the two code points in question are the current input code point and the next input code point, in that order.

Note: This algorithm will not consume any additional code point.

If the first code point is not U+005C REVERSE SOLIDUS (\), return false.

Otherwise, if the second code point is a newline, return false.

Otherwise, return true.

4.3.9. Check if three code points would start an ident sequence

This section describes how to check if three code points would start an ident sequence. The algorithm described here can be called explicitly with three code points, or can be called with the input stream itself. In the latter case, the three code points in question are the current input code point and the next two input code points, in that order.

Note: This algorithm will not consume any additional code points.

Look at the first code point:

U+002D HYPHEN-MINUS

If the second code point is an ident-start code point or a U+002D HYPHEN-MINUS, or the second and third code points are a valid escape, return true. Otherwise, return false.

ident-start code point

Return true.

U+005C REVERSE SOLIDUS (\)

If the first and second code points are a valid escape, return true. Otherwise, return false.

anything else

Return false.

4.3.10. Check if three code points would start a number

This section describes how to check if three code points would start a number. The algorithm described here can be called explicitly with three code points, or can be called with the input stream itself. In the latter case, the three code points in question are the current input code point and the next two input code points, in that order.

Note: This algorithm will not consume any additional code points.

Look at the first code point:

U+002B PLUS SIGN (+)

U+002D HYPHEN-MINUS (-)

If the second code point is a digit, return true.

Otherwise, if the second code point is a U+002E FULL STOP (.) and the third code point is a digit, return true.

Otherwise, return false.

U+002E FULL STOP (.)

If the second code point is a digit, return true. Otherwise, return false.

digit

Return true.

anything else

Return false.

4.3.11. Consume an ident sequence

This section describes how to consume an ident sequence from a stream of code points. It returns a string containing the largest name that can be formed from adjacent code points in the stream, starting from the first.

Note: This algorithm does not do the verification of the first few code points that are necessary to ensure the returned code points would constitute an <ident-token>. If that is the intended use, ensure that the stream starts with an ident sequence before calling this algorithm.

Let result initially be an empty string.

Repeatedly consume the next input code point from the stream:

ident code point

Append the code point to result.

the stream starts with a valid escape

Consume an escaped code point. Append the returned code point to result.

anything else

Reconsume the current input code point. Return result.

4.3.12. Consume a number

This section describes how to consume a number from a stream of code points. It returns a numeric value, and a type which is either "integer" or "number".

Note: This algorithm does not do the verification of the first few code points that are necessary to ensure a number can be obtained from the stream. Ensure that the stream starts with a number before calling this algorithm.

Execute the following steps in order:

1. Initially set type to "integer". Let repr be the empty string.
2. If the next input code point is U+002B PLUS SIGN (+) or U+002D HYPHEN-MINUS (-), consume it and append it to repr.
3. While the next input code point is a digit, consume it and append it to repr.
4. If the next 2 input code points are U+002E FULL STOP (.) followed by a digit, then:
   1. Consume them.
   2. Append them to repr.
   3. Set type to "number".
   4. While the next input code point is a digit, consume it and append it to repr.
5. If the next 2 or 3 input code points are U+0045 LATIN CAPITAL LETTER E (E) or U+0065 LATIN SMALL LETTER E (e), optionally followed by U+002D HYPHEN-MINUS (-) or U+002B PLUS SIGN (+), followed by a digit, then:
   1. Consume them.
   2. Append them to repr.
   3. Set type to "number".
   4. While the next input code point is a digit, consume it and append it to repr.
6. Convert repr to a number, and set the value to the returned value.
7. Return value and type.

4.3.13. Convert a string to a number

This section describes how to convert a string to a number. It returns a number.

Note: This algorithm does not do any verification to ensure that the string contains only a number. Ensure that the string contains only a valid CSS number before calling this algorithm.

Divide the string into seven components, in order from left to right:

1. A sign: a single U+002B PLUS SIGN (+) or U+002D HYPHEN-MINUS (-), or the empty string. Let s be the number -1 if the sign is U+002D HYPHEN-MINUS (-); otherwise, let s be the number 1.
2. An integer part: zero or more digits. If there is at least one digit, let i be the number formed by interpreting the digits as a base-10 integer; otherwise, let i be the number 0.
3. A decimal point: a single U+002E FULL STOP (.), or the empty string.
4. A fractional part: zero or more digits. If there is at least one digit, let f be the number formed by interpreting the digits as a base-10 integer and d be the number of digits; otherwise, let f and d be the number 0.
5. An exponent indicator: a single U+0045 LATIN CAPITAL LETTER E (E) or U+0065 LATIN SMALL LETTER E (e), or the empty string.
6. An exponent sign: a single U+002B PLUS SIGN (+) or U+002D HYPHEN-MINUS (-), or the empty string. Let t be the number -1 if the sign is U+002D HYPHEN-MINUS (-); otherwise, let t be the number 1.
7. An exponent: zero or more digits. If there is at least one digit, let e be the number formed by interpreting the digits as a base-10 integer; otherwise, let e be the number 0.

Return the number s·(i + f·10-d)·10te.

4.3.14. Consume the remnants of a bad url

This section describes how to consume the remnants of a bad url from a stream of code points, "cleaning up" after the tokenizer realizes that it’s in the middle of a <bad-url-token> rather than a <url-token>. It returns nothing; its sole use is to consume enough of the input stream to reach a recovery point where normal tokenizing can resume.

Repeatedly consume the next input code point from the stream:

U+0029 RIGHT PARENTHESIS ())

EOF

Return.

the input stream starts with a valid escape

Consume an escaped code point. This allows an escaped right parenthesis ("\)") to be encountered without ending the <bad-url-token>. This is otherwise identical to the "anything else" clause.

anything else

Do nothing.

5. Parsing

The input to the parsing stage is a stream or list of tokens from the tokenization stage. The output depends on how the parser is invoked, as defined by the entry points listed later in this section. The parser output can consist of at-rules, qualified rules, and/or declarations.

The parser’s output is constructed according to the fundamental syntax of CSS, without regards for the validity of any specific item. Implementations may check the validity of items as they are returned by the various parser algorithms and treat the algorithm as returning nothing if the item was invalid according to the implementation’s own grammar knowledge, or may construct a full tree as specified and "clean up" afterwards by removing any invalid items.

The items that can appear in the tree are:

at-rule

An at-rule has a name, a prelude consisting of a list of component values, and an optional block consisting of a simple {} block.

Note: This specification places no limits on what an at-rule’s block may contain. Individual at-rules must define whether they accept a block, and if so, how to parse it (preferably using one of the parser algorithms or entry points defined in this specification).

qualified rule

A qualified rule has a prelude consisting of a list of component values, and a block consisting of a simple {} block.

Note: Most qualified rules will be style rules, where the prelude is a selector [SELECT] and the block a list of declarations.

declaration

Conceptually, declarations are a particular instance of associating a property or descriptor name with a value. Syntactically, a declaration has a name, a value consisting of a list of component values, and an important flag which is initially unset.

Declarations are further categorized as property declarations or descriptor declarations, with the former setting CSS properties and appearing most often in qualified rules and the latter setting CSS descriptors, which appear only in at-rules. (This categorization does not occur at the Syntax level; instead, it is a product of where the declaration appears, and is defined by the respective specifications defining the given rule.)

component value

A component value is one of the preserved tokens, a function, or a simple block.

preserved tokens

Any token produced by the tokenizer except for <function-token>s, <{-token>s, <(-token>s, and <[-token>s.

Note: The non-preserved tokens listed above are always consumed into higher-level objects, either functions or simple blocks, and so never appear in any parser output themselves.

Note: The tokens <}-token>s, <)-token>s, <]-token>, <bad-string-token>, and <bad-url-token> are always parse errors, but they are preserved in the token stream by this specification to allow other specs, such as Media Queries, to define more fine-grained error-handling than just dropping an entire declaration or block.

function

A function has a name and a value consisting of a list of component values.

simple block

{}-block

[]-block

()-block

A simple block has an associated token (either a <[-token>, <(-token>, or <{-token>) and a value consisting of a list of component values.

{}-block, []-block, and ()-block refer specifically to a simple block with that corresponding associated token.

5.1. Parser Railroad Diagrams

This section is non-normative.

This section presents an informative view of the parser, in the form of railroad diagrams.

These diagrams are informative and incomplete; they describe the grammar of "correct" stylesheets, but do not describe error-handling at all. They are provided solely to make it easier to get an intuitive grasp of the syntax.

Stylesheet

Rule list

At-rule

Qualified rule

Declaration list

Declaration

!important

Component value

{} block

() block

[] block

Function block

5.2. Definitions

current input token

The token or component value currently being operated on, from the list of tokens produced by the tokenizer.

next input token

The token or component value following the current input token in the list of tokens produced by the tokenizer. If there isn’t a token following the current input token, the next input token is an <EOF-token>.

<EOF-token>

A conceptual token representing the end of the list of tokens. Whenever the list of tokens is empty, the next input token is always an <EOF-token>.

consume the next input token

Let the current input token be the current next input token, adjusting the next input token accordingly.

reconsume the current input token

The next time an algorithm instructs you to consume the next input token, instead do nothing (retain the current input token unchanged).

5.3. Parser Entry Points

The algorithms defined in this section produce high-level CSS objects from lists of CSS tokens.

Note: Other specs can define additional entry points for their own purposes.

5.3.1. Parse something according to a CSS grammar

It is often desirable to parse a string or token list to see if it matches some CSS grammar, and if it does, to destructure it according to the grammar. This section provides a generic hook for this kind of operation. It should be invoked like "parse foo as a CSS <color>", or similar.

This algorithm returns either failure, if the input does not match the provided grammar, or the result of parsing the input according to the grammar, which is an unspecified structure corresponding to the provided grammar specification. The return value must only be interacted with by specification prose, where the representation ambiguity is not problematic. If it is meant to be exposed outside of spec language, the spec using the result must explicitly translate it into a well-specified representation, such as, for example, by invoking a CSS serialization algorithm (like "serialize as a CSS <string> value").

Note: This algorithm, and parse a comma-separated list according to a CSS grammar, are usually the only parsing algorithms other specs will want to call. The remaining parsing algorithms are meant mostly for [CSSOM] and related "explicitly constructing CSS structures" cases. Consult the CSSWG for guidance first if you think you need to use one of the other algorithms.

5.3.2. Parse A Comma-Separated List According To A CSS Grammar

While one can definitely parse a value according to a grammar with commas in it, if any part of the value fails to parse, the entire thing doesn’t parse, and returns failure.

Sometimes that’s what’s desired (such as in list-valued CSS properties); other times, it’s better to let each comma-separated sub-part of the value parse separately, dealing with the parts that parse successfully one way, and the parts that fail to parse another way (typically ignoring them, such as in <img sizes>).

This algorithm provides an easy hook to accomplish exactly that. It returns a list of values split by "top-level" commas, where each values is either failure (if it failed to parse) or the result of parsing (an unspecified structure, as described in the parse algorithm).

5.3.3. Parse a stylesheet

5.3.4. Parse a list of rules

5.3.5. Parse a rule

5.3.6. Parse a declaration

Note: Unlike "Parse a list of declarations", this parses only a declaration and not an at-rule.

5.3.7. Parse a style block’s contents

Note: This algorithm parses the contents of style rules, which need to allow nested style rules and other at-rules. If you don’t need nested style rules, such as in @page or in @keyframes child rules, use parse a list of declarations.

5.3.8. Parse a list of declarations

Note: Despite the name, this actually parses a mixed list of declarations and at-rules, as CSS 2.1 does for @page. Unexpected at-rules (which could be all of them, in a given context) are invalid and will be ignored by the consumer.

Note: This algorithm does not handle nested style rules. If your use requires that, use parse a style block’s contents.

5.3.9. Parse a component value

5.3.10. Parse a list of component values

5.3.11. Parse a comma-separated list of component values

5.4. Parser Algorithms

The following algorithms comprise the parser. They are called by the parser entry points above.

These algorithms may be called with a list of either tokens or of component values. (The difference being that some tokens are replaced by functions and simple blocks in a list of component values.) Similar to how the input stream returned EOF code points to represent when it was empty during the tokenization stage, the lists in this stage must return an <EOF-token> when the next token is requested but they are empty.

An algorithm may be invoked with a specific list, in which case it consumes only that list (and when that list is exhausted, it begins returning <EOF-token>s). Otherwise, it is implicitly invoked with the same list as the invoking algorithm.

5.4.1. Consume a list of rules

To consume a list of rules, given a top-level flag:

Create an initially empty list of rules.

Repeatedly consume the next input token:

<whitespace-token>

Do nothing.

<EOF-token>

Return the list of rules.

<CDO-token>

<CDC-token>

If the top-level flag is set, do nothing.

Otherwise, reconsume the current input token. Consume a qualified rule. If anything is returned, append it to the list of rules.

<at-keyword-token>

Reconsume the current input token. Consume an at-rule, and append the returned value to the list of rules.

anything else

Reconsume the current input token. Consume a qualified rule. If anything is returned, append it to the list of rules.

5.4.2. Consume an at-rule

To consume an at-rule:

Consume the next input token. Create a new at-rule with its name set to the value of the current input token, its prelude initially set to an empty list, and its value initially set to nothing.

Repeatedly consume the next input token:

<semicolon-token>

Return the at-rule.

<EOF-token>

This is a parse error. Return the at-rule.

<{-token>

Consume a simple block and assign it to the at-rule’s block. Return the at-rule.

simple block with an associated token of <{-token>

Assign the block to the at-rule’s block. Return the at-rule.

anything else

Reconsume the current input token. Consume a component value. Append the returned value to the at-rule’s prelude.

5.4.3. Consume a qualified rule

To consume a qualified rule:

Create a new qualified rule with its prelude initially set to an empty list, and its value initially set to nothing.

Repeatedly consume the next input token:

<EOF-token>

This is a parse error. Return nothing.

<{-token>

Consume a simple block and assign it to the qualified rule’s block. Return the qualified rule.

simple block with an associated token of <{-token>

Assign the block to the qualified rule’s block. Return the qualified rule.

anything else

Reconsume the current input token. Consume a component value. Append the returned value to the qualified rule’s prelude.

5.4.4. Consume a style block’s contents

To consume a style block’s contents:

Create an initially empty list of declarations decls, and an initially empty list of rules rules.

Repeatedly consume the next input token:

<whitespace-token>

<semicolon-token>

Do nothing.

<EOF-token>

Extend decls with rules, then return decls.

<at-keyword-token>

Reconsume the current input token. Consume an at-rule, and append the result to rules.

<ident-token>

Initialize a temporary list initially filled with the current input token. As long as the next input token is anything other than a <semicolon-token> or <EOF-token>, consume a component value and append it to the temporary list. Consume a declaration from the temporary list. If anything was returned, append it to decls.

<delim-token> with a value of "&" (U+0026 AMPERSAND)

Reconsume the current input token. Consume a qualified rule. If anything was returned, append it to rules.

anything else

This is a parse error. Reconsume the current input token. As long as the next input token is anything other than a <semicolon-token> or <EOF-token>, consume a component value and throw away the returned value.

5.4.5. Consume a list of declarations

To consume a list of declarations:

Create an initially empty list of declarations.

Repeatedly consume the next input token:

<whitespace-token>

<semicolon-token>

Do nothing.

<EOF-token>

Return the list of declarations.

<at-keyword-token>

Reconsume the current input token. Consume an at-rule. Append the returned rule to the list of declarations.

<ident-token>

Initialize a temporary list initially filled with the current input token. As long as the next input token is anything other than a <semicolon-token> or <EOF-token>, consume a component value and append it to the temporary list. Consume a declaration from the temporary list. If anything was returned, append it to the list of declarations.

anything else

This is a parse error. Reconsume the current input token. As long as the next input token is anything other than a <semicolon-token> or <EOF-token>, consume a component value and throw away the returned value.

5.4.6. Consume a declaration

Note: This algorithm assumes that the next input token has already been checked to be an <ident-token>.

To consume a declaration:

Consume the next input token. Create a new declaration with its name set to the value of the current input token and its value initially set to an empty list.

1. While the next input token is a <whitespace-token>, consume the next input token.
2. If the next input token is anything other than a <colon-token>, this is a parse error. Return nothing.

   Otherwise, consume the next input token.

3. While the next input token is a <whitespace-token>, consume the next input token.
4. As long as the next input token is anything other than an <EOF-token>, consume a component value and append it to the declaration’s value.
5. If the last two non-<whitespace-token>s in the declaration’s value are a <delim-token> with the value "!" followed by an <ident-token> with a value that is an ASCII case-insensitive match for "important", remove them from the declaration’s value and set the declaration’s important flag to true.
6. While the last token in the declaration’s value is a <whitespace-token>, remove that token.
7. Return the declaration.

5.4.7. Consume a component value

To consume a component value:

Consume the next input token.

If the current input token is a <{-token>, <[-token>, or <(-token>, consume a simple block and return it.

Otherwise, if the current input token is a <function-token>, consume a function and return it.

Otherwise, return the current input token.

5.4.8. Consume a simple block

Note: This algorithm assumes that the current input token has already been checked to be an <{-token>, <[-token>, or <(-token>.

To consume a simple block:

The ending token is the mirror variant of the current input token. (E.g. if it was called with <[-token>, the ending token is <]-token>.)

Create a simple block with its associated token set to the current input token and with its value initially set to an empty list.

Repeatedly consume the next input token and process it as follows:

ending token

Return the block.

<EOF-token>

This is a parse error. Return the block.

anything else

Reconsume the current input token. Consume a component value and append it to the value of the block.

Note: CSS has an unfortunate syntactic ambiguity between blocks that can contain declarations and blocks that can contain qualified rules, so any "consume" algorithms that handle rules will initially use this more generic algorithm rather than the more specific consume a list of declarations or consume a list of rules algorithms. These more specific algorithms are instead invoked when grammars are applied, depending on whether it contains a <declaration-list> or a <rule-list>/<stylesheet>.

5.4.9. Consume a function

Note: This algorithm assumes that the current input token has already been checked to be a <function-token>.

To consume a function:

Create a function with its name equal to the value of the current input token and with its value initially set to an empty list.

Repeatedly consume the next input token and process it as follows:

<)-token>

Return the function.

<EOF-token>

This is a parse error. Return the function.

anything else

Reconsume the current input token. Consume a component value and append the returned value to the function’s value.

6. The An+B microsyntax

Several things in CSS, such as the :nth-child() pseudoclass, need to indicate indexes in a list. The An+B microsyntax is useful for this, allowing an author to easily indicate single elements or all elements at regularly-spaced intervals in a list.

The An+B notation defines an integer step (A) and offset (B), and represents the An+Bth elements in a list, for every positive integer or zero value of n, with the first element in the list having index 1 (not 0).

For values of A and B greater than 0, this effectively divides the list into groups of A elements (the last group taking the remainder), and selecting the Bth element of each group.

The An+B notation also accepts the even and odd keywords, which have the same meaning as 2n and 2n+1, respectively.

2n+0   /* represents all of the even elements in the list */

even   /* same */

4n+1   /* represents the 1st, 5th, 9th, 13th, etc. elements in the list */

The values of A and B can be negative, but only the positive results of An+B, for n ≥ 0, are used.

-1n+6   /* represents the first 6 elements of the list */

-4n+10  /* represents the 2nd, 6th, and 10th elements of the list */

If both A and B are 0, the pseudo-class represents no element in the list.

6.1. Informal Syntax Description

This section is non-normative.

When A is 0, the An part may be omitted (unless the B part is already omitted). When An is not included and B is non-negative, the + sign before B (when allowed) may also be omitted. In this case the syntax simplifies to just B.

0n+5   /* represents the 5th element in the list */

5      /* same */

When A is 1 or -1, the 1 may be omitted from the rule.

1n+0   /* represents all elements in the list */

n+0    /* same */

n      /* same */

If B is 0, then every Ath element is picked. In such a case, the +B (or -B) part may be omitted unless the A part is already omitted.

2n+0   /* represents every even element in the list */

2n     /* same */

When B is negative, its minus sign replaces the + sign.

3n-6

3n + -6

Whitespace is permitted on either side of the + or - that separates the An and B parts when both are present.

3n + 1

+3n - 2

-n+ 6

+6

3 n

+ 2n

+ 2

6.2. The <an+b> type

The An+B notation was originally defined using a slightly different tokenizer than the rest of CSS, resulting in a somewhat odd definition when expressed in terms of CSS tokens. This section describes how to recognize the An+B notation in terms of CSS tokens (thus defining the <an+b> type for CSS grammar purposes), and how to interpret the CSS tokens to obtain values for A and B.

The <an+b> type is defined (using the Value Definition Syntax in the Values & Units spec) as:

<an+b> =
  odd | even |
  <integer> |

  <n-dimension> |
  '+'?† n |
  -n |

  <ndashdigit-dimension> |
  '+'?† <ndashdigit-ident> |
  <dashndashdigit-ident> |

  <n-dimension> <signed-integer> |
  '+'?† n <signed-integer> |
  -n <signed-integer> |

  <ndash-dimension> <signless-integer> |
  '+'?† n- <signless-integer> |
  -n- <signless-integer> |

  <n-dimension> ['+' | '-'] <signless-integer> |
  '+'?† n ['+' | '-'] <signless-integer> |
  -n ['+' | '-'] <signless-integer>

where:

• <n-dimension> is a <dimension-token> with its type flag set to "integer", and a unit that is an ASCII case-insensitive match for "n"
• <ndash-dimension> is a <dimension-token> with its type flag set to "integer", and a unit that is an ASCII case-insensitive match for "n-"
• <ndashdigit-dimension> is a <dimension-token> with its type flag set to "integer", and a unit that is an ASCII case-insensitive match for "n-*", where "*" is a series of one or more digits
• <ndashdigit-ident> is an <ident-token> whose value is an ASCII case-insensitive match for "n-*", where "*" is a series of one or more digits
• <dashndashdigit-ident> is an <ident-token> whose value is an ASCII case-insensitive match for "-n-*", where "*" is a series of one or more digits
• <integer> is a <number-token> with its type flag set to "integer"
• <signed-integer> is a <number-token> with its type flag set to "integer", and whose representation starts with "+" or "-"
• <signless-integer> is a <number-token> with its type flag set to "integer", and whose representation starts with a digit

^†: When a plus sign (+) precedes an ident starting with "n", as in the cases marked above, there must be no whitespace between the two tokens, or else the tokens do not match the above grammar. Whitespace is valid (and ignored) between any other two tokens.

The clauses of the production are interpreted as follows:

odd

A is 2, B is 1.

even

A is 2, B is 0.

<integer>

A is 0, B is the integer’s value.

<n-dimension>

'+'? n

-n

A is the dimension’s value, 1, or -1, respectively. B is 0.

<ndashdigit-dimension>

'+'? <ndashdigit-ident>

A is the dimension’s value or 1, respectively. B is the dimension’s unit or ident’s value, respectively, with the first code point removed and the remainder interpreted as a base-10 number. B is negative.

<dashndashdigit-ident>

A is -1. B is the ident’s value, with the first two code points removed and the remainder interpreted as a base-10 number. B is negative.

<n-dimension> <signed-integer>

'+'? n <signed-integer>

-n <signed-integer>

A is the dimension’s value, 1, or -1, respectively. B is the integer’s value.

<ndash-dimension> <signless-integer>

'+'? n- <signless-integer>

-n- <signless-integer>

A is the dimension’s value, 1, or -1, respectively. B is the negation of the integer’s value.

<n-dimension> ['+' | '-'] <signless-integer>

'+'? n ['+' | '-'] <signless-integer>

-n ['+' | '-'] <signless-integer>

A is the dimension’s value, 1, or -1, respectively. B is the integer’s value. If a '-' was provided between the two, B is instead the negation of the integer’s value.

7. The Unicode-Range microsyntax

Some constructs, such as the unicode-range descriptor for the @font-face rule, need a way to describe one or more unicode code points. The <urange> production represents a range of one or more unicode code points.

Informally, the <urange> production has three forms:

U+0001

Defines a range consisting of a single code point, in this case the code point "1".

U+0001-00ff

Defines a range of codepoints between the first and the second value inclusive, in this case the range between "1" and "ff" (255 in decimal) inclusive.

U+00??

Defines a range of codepoints where the "?" characters range over all hex digits, in this case defining the same as the value U+0000-00ff.

In each form, a maximum of 6 digits is allowed for each hexadecimal number (if you treat "?" as a hexadecimal digit).

7.1. The <urange> type

The <urange> notation was originally defined as a primitive token in CSS, but it is used very rarely, and collides with legitimate <ident-token>s in confusing ways. This section describes how to recognize the <urange> notation in terms of existing CSS tokens, and how to interpret it as a range of unicode codepoints.

Note: The syntax described here is intentionally very low-level, and geared toward implementors. Authors should instead read the informal syntax description in the previous section, as it contains all information necessary to use <urange>, and is actually readable.

The <urange> type is defined (using the Value Definition Syntax in the Values & Units spec) as:

<urange> =
  u '+' <ident-token> '?'* |
  u <dimension-token> '?'* |
  u <number-token> '?'* |
  u <number-token> <dimension-token> |
  u <number-token> <number-token> |
  u '+' '?'+

In this production, no whitespace can occur between any of the tokens.

The <urange> production represents a range of one or more contiguous unicode code points as a start value and an end value, which are non-negative integers. To interpret the production above into a range, execute the following steps in order:

1. Skipping the first u token, concatenate the representations of all the tokens in the production together. Let this be text.

2. If the first character of text is U+002B PLUS SIGN, consume it. Otherwise, this is an invalid <urange>, and this algorithm must exit.

3. Consume as many hex digits from text as possible, then consume as many U+003F QUESTION MARK (?) code points as possible. If zero code points were consumed, or more than six code points were consumed, this is an invalid <urange>, and this algorithm must exit.

   If any U+003F QUESTION MARK (?) code points were consumed, then:

   1. If there are any code points left in text, this is an invalid <urange>, and this algorithm must exit.

   2. Interpret the consumed code points as a hexadecimal number, with the U+003F QUESTION MARK (?) code points replaced by U+0030 DIGIT ZERO (0) code points. This is the start value.

   3. Interpret the consumed code points as a hexadecimal number again, with the U+003F QUESTION MARK (?) code points replaced by U+0046 LATIN CAPITAL LETTER F (F) code points. This is the end value.

   4. Exit this algorithm.

   Otherwise, interpret the consumed code points as a hexadecimal number. This is the start value.

4. If there are no code points left in text, the end value is the same as the start value. Exit this algorithm.

5. If the next code point in text is U+002D HYPHEN-MINUS (-), consume it. Otherwise, this is an invalid <urange>, and this algorithm must exit.

6. Consume as many hex digits as possible from text.

   If zero hex digits were consumed, or more than 6 hex digits were consumed, this is an invalid <urange>, and this algorithm must exit. If there are any code points left in text, this is an invalid <urange>, and this algorithm must exit.

7. Interpret the consumed code points as a hexadecimal number. This is the end value.

To determine what codepoints the <urange> represents:

1. If end value is greater than the maximum allowed code point, the <urange> is invalid and a syntax error.

2. If start value is greater than end value, the <urange> is invalid and a syntax error.

3. Otherwise, the <urange> represents a contiguous range of codepoints from start value to end value, inclusive.

Note: The syntax of <urange> is intentionally fairly wide; its patterns capture every possible token sequence that the informal syntax can generate. However, it requires no whitespace between its constituent tokens, which renders it fairly safe to use in practice. Even grammars which have a <urange> followed by a <number> or <dimension> (which might appear to be ambiguous if an author specifies the <urange> with the ''u <number>'' clause) are actually quite safe, as an author would have to intentionally separate the <urange> and the <number>/<dimension> with a comment rather than whitespace for it to be ambiguous. Thus, while it’s possible for authors to write things that are parsed in confusing ways, the actual code they’d have to write to cause the confusion is, itself, confusing and rare.

8. Defining Grammars for Rules and Other Values

CSS Values 4 § 2 Value Definition Syntax defines how to specify a grammar for properties. This section extends those definitions to also allow specifying a grammar for rules.

Non-terminals representing the entire grammar of an at-rule are written as an @ character followed by the at-rule’s name, between < and >, e.g. <@media> to represent the @media rule.

The bracketed range notation can be used on any of the numeric token non-terminals.

Several types of tokens are written literally, without quotes:

• <ident-token>s (such as auto, disc, etc), which are simply written as their value.
• <at-keyword-token>s, which are written as an @ character followed by the token’s value, like @media.
• <function-token>s, which are written as the function name followed by a ( character, like translate(.
• The <colon-token> (written as :), <comma-token> (written as ,), <semicolon-token> (written as ;), <(-token>, <)-token>, <{-token>, and <}-token>s.

Tokens match if their value is a match for the value defined in the grammar. Unless otherwise specified, all matches are ASCII case-insensitive.

Note: Although it is possible, with escaping, to construct an <ident-token> whose value ends with ( or starts with @, such a tokens is not a <function-token> or an <at-keyword-token> and does not match corresponding grammar definitions.

<delim-token>s are written with their value enclosed in single quotes. For example, a <delim-token> containing the "+" code point is written as '+'. Similarly, the <[-token> and <]-token>s must be written in single quotes, as they’re used by the syntax of the grammar itself to group clauses. <whitespace-token> is never indicated in the grammar; <whitespace-token>s are allowed before, after, and between any two tokens, unless explicitly specified otherwise in prose definitions. (For example, if the prelude of a rule is a selector, whitespace is significant.)

When defining a function or a block, the ending token must be specified in the grammar, but if it’s not present in the eventual token stream, it still matches.

translateX( <translation-value> )

.foo { transform: translate(50px

8.1. Defining Block Contents: the <declaration-list>, <rule-list>, and <stylesheet> productions

The CSS parser is agnostic as to the contents of blocks, such as those that come at the end of some at-rules. Defining the generic grammar of the blocks in terms of tokens is non-trivial, but there are dedicated and unambiguous algorithms defined for parsing this.

The <style-block> production represents the contents of a style rule’s block. It may only be used in grammars as the sole value in a block, and represents that the contents of the block must be parsed using the consume a style block’s contents algorithm.

The <declaration-list> production represents a list of declarations. It may only be used in grammars as the sole value in a block, and represents that the contents of the block must be parsed using the consume a list of declarations algorithm.

Similarly, the <rule-list> production represents a list of rules, and may only be used in grammars as the sole value in a block. It represents that the contents of the block must be parsed using the consume a list of rules algorithm.

Finally, the <stylesheet> production represents a list of rules. It is identical to <rule-list>, except that blocks using it default to accepting all rules that aren’t otherwise limited to a particular context.

@font-face { <declaration-list> }

@keyframes <keyframes-name> { <rule-list> }

For rules that use <style-block> or <declaration-list>, the spec for the rule must define which properties, descriptors, and/or at-rules are valid inside the rule; this may be as simple as saying "The @foo rule accepts the properties/descriptors defined in this specification/section.", and extension specs may simply say "The @foo rule additionally accepts the following properties/descriptors.". Any declarations or at-rules found inside the block that are not defined as valid must be removed from the rule’s value.

Within a <style-block> or <declaration-list>, !important is automatically invalid on any descriptors. If the rule accepts properties, the spec for the rule must define whether the properties interact with the cascade, and with what specificity. If they don’t interact with the cascade, properties containing !important are automatically invalid; otherwise using !important is valid and causes the declaration to be important for the purposes of the cascade. See [CSS-CASCADE-3].

For rules that use <rule-list>, the spec for the rule must define what types of rules are valid inside the rule, same as <declaration-list>, and unrecognized rules must similarly be removed from the rule’s value.

<keyframe-rule> = <keyframe-selector> { <declaration-list> }

For rules that use <stylesheet>, all rules are allowed by default, but the spec for the rule may define what types of rules are invalid inside the rule.

@media <media-query-list> { <stylesheet> }

8.2. Defining Arbitrary Contents: the <declaration-value> and <any-value> productions

In some grammars, it is useful to accept any reasonable input in the grammar, and do more specific error-handling on the contents manually (rather than simply invalidating the construct, as grammar mismatches tend to do).

For example, custom properties allow any reasonable value, as they can contain arbitrary pieces of other CSS properties, or be used for things that aren’t part of existing CSS at all. For another example, the <general-enclosed> production in Media Queries defines the bounds of what future syntax MQs will allow, and uses special logic to deal with "unknown" values.

To aid in this, two additional productions are defined:

The <declaration-value> production matches any sequence of one or more tokens, so long as the sequence does not contain <bad-string-token>, <bad-url-token>, unmatched <)-token>, <]-token>, or <}-token>, or top-level <semicolon-token> tokens or <delim-token> tokens with a value of "!". It represents the entirety of what a valid declaration can have as its value.

The <any-value> production is identical to <declaration-value>, but also allows top-level <semicolon-token> tokens and <delim-token> tokens with a value of "!". It represents the entirety of what valid CSS can be in any context.

9. CSS stylesheets

To parse a CSS stylesheet, first parse a stylesheet. Interpret all of the resulting top-level qualified rules as style rules, defined below.

If any style rule is invalid, or any at-rule is not recognized or is invalid according to its grammar or context, it’s a parse error. Discard that rule.

9.1. Style rules

A style rule is a qualified rule that associates a selector list with a list of property declarations and possibly a list of nested rules. They are also called rule sets in [CSS2]. CSS Cascading and Inheritance [CSS-CASCADE-3] defines how the declarations inside of style rules participate in the cascade.

The prelude of the qualified rule is parsed as a <selector-list>. If this returns failure, the entire style rule is invalid.

The content of the qualified rule’s block is parsed as a style block’s contents. Unless defined otherwise by another specification or a future level of this specification, at-rules in that list are invalid and must be ignored.

Note: [CSS-NESTING-1] defines that @nest and conditional group rules are allowed inside of style rules.

Declarations for an unknown CSS property or whose value does not match the syntax defined by the property are invalid and must be ignored. The validity of the style rule’s contents have no effect on the validity of the style rule itself. Unless otherwise specified, property names are ASCII case-insensitive.

Note: The names of Custom Properties [CSS-VARIABLES] are case-sensitive.

Qualified rules at the top-level of a CSS stylesheet are style rules. Qualified rules in other contexts may or may not be style rules, as defined by the context.

For example, qualified rules inside @media rules [CSS3-CONDITIONAL] are style rules, but qualified rules inside @keyframes rules [CSS3-ANIMATIONS] are not.

9.2. At-rules

An at-rule is a rule that begins with an at-keyword, and can thus be distinguished from style rules in the same context.

At-rules are used to:

• group and structure style rules and other at-rules such as in conditional group rules

• declare style information that is not associated with a particular element, such as defining counter styles

• manage syntactic constructs such as imports and namespaces keyword mappings

• and serve other miscellaneous purposes not served by a style rule

At-rules take many forms, depending on the specific rule and its purpose, but broadly speaking there are two kinds: statement at-rules which are simpler constructs that end in a semicolon, and block at-rules which end in a {}-block that can contain nested qualified rules, at-rules, or declarations.

Block at-rules will typically contain a collection of (generic or at-rule–specific) at-rules, qualified rules, and/or descriptor declarations subject to limitations defined by the at-rule. Descriptors are similar to properties (and are declared with the same syntax) but are associated with a particular type of at-rule rather than with elements and boxes in the tree.

9.3. The @charset Rule

The algorithm used to determine the fallback encoding for a stylesheet looks for a specific byte sequence as the very first few bytes in the file, which has the syntactic form of an at-rule named "@charset".

However, there is no actual at-rule named @charset. When a stylesheet is actually parsed, any occurrences of an @charset rule must be treated as an unrecognized rule, and thus dropped as invalid when the stylesheet is grammar-checked.

Note: In CSS 2.1, @charset was a valid rule. Some legacy specs may still refer to a @charset rule, and explicitly talk about its presence in the stylesheet.

10. Serialization

The tokenizer described in this specification does not produce tokens for comments, or otherwise preserve them in any way. Implementations may preserve the contents of comments and their location in the token stream. If they do, this preserved information must have no effect on the parsing step.

This specification does not define how to serialize CSS in general, leaving that task to the [CSSOM] and individual feature specifications. In particular, the serialization of comments and whitespace is not defined.

The only requirement for serialization is that it must "round-trip" with parsing, that is, parsing the stylesheet must produce the same data structures as parsing, serializing, and parsing again, except for consecutive <whitespace-token>s, which may be collapsed into a single token.

Note: This exception can exist because CSS grammars always interpret any amount of whitespace as identical to a single space.

10.1. Serializing <an+b>

11. Privacy and Security Considerations

This specification introduces no new privacy concerns.

This specification improves security, in that CSS parsing is now unambiguously defined for all inputs.

Insofar as old parsers, such as whitelists/filters, parse differently from this specification, they are somewhat insecure, but the previous parsing specification left a lot of ambiguous corner cases which browsers interpreted differently, so those filters were potentially insecure already, and this specification does not worsen the situation.

12. Changes

This section is non-normative.

12.1. Changes from the 16 August 2019 Candidate Recommendation

The following substantive changes were made:

• Added a new § 5.3.2 Parse A Comma-Separated List According To A CSS Grammar algorithm.

• Added a new § 5.3.7 Parse a style block’s contents algorithm and a corresponding <style-block> production, and defined that style rules use it.

• Aligned parse a stylesheet with the Fetch-related shenanigans. (See commit.)

  To parse a stylesheet from an input given an optional url location :

  1. ...
  2. Create a new stylesheet , with its location set to location (or null, if location was not passed).
  3. ...

The following editorial changes were made:

• Added § 9.2 At-rules to provide definitions for at-rules, statement at-rules, block at-rules, and descriptors. (5633)

• Improved the definition text for declaration, and added definitions for property declarations and descriptor declarations.

• Switched to consistently refer to ident sequence, rather than sometimes using the term “name”.

• Explicitly named several of the pre-tokenizing processes, and explicitly referred to them in the parsing entry points (rather than relying on a blanket "do X at the start of these algorithms" statement).

• Added more entries to the "put a comment between them" table, to properly handle the fact that idents can now start with --. (6874)

12.2. Changes from the 20 February 2014 Candidate Recommendation

The following substantive changes were made:

• Removed <unicode-range-token>, in favor of creating a <urange> production.

• url() functions that contain a string are now parsed as normal <function-token>s. url() functions that contain "raw" URLs are still specially parsed as <url-token>s.

• Fixed a bug in the "Consume a URL token" algorithm, where it didn’t consume the quote character starting a string before attempting to consume the string.

• Fixed a bug in several of the parser algorithms related to the current/next input token and things getting consumed early/late.

• Fix several bugs in the tokenization and parsing algorithms.

• Change the definition of ident-like tokens to allow "--" to start an ident. As part of this, rearrange the ordering of the clauses in the "-" step of consume a token so that <CDC-token>s are recognized as such instead of becoming a -- <ident-token>.

• Don’t serialize the digit in an <an+b> when A is 1 or -1.

• Define all tokens to have a representation.

• Fixed minor bug in check if two code points are a valid escape—a \ followed by an EOF is now correctly reported as not a valid escape. A final \ in a stylesheet now just emits itself as a <delim-token>.

• @charset is no longer a valid CSS rule (there’s just an encoding declaration that looks like a rule named @charset)

• Trimmed whitespace from the beginning/ending of a declaration’s value during parsing.

• Removed the Selectors-specific tokens, per WG resolution.

• Filtered surrogates from the input stream, per WG resolution. Now the entire specification operates only on scalar values.

The following editorial changes were made:

• The "Consume a string token" algorithm was changed to allow calling it without specifying an explicit ending token, so that it uses the current input token instead. The three call-sites of the algorithm were changed to use that form.

• Minor editorial restructuring of algorithms.

• Added the parse and parse a comma-separated list of component values API entry points.

• Added the <declaration-value> and <any-value> productions.

• Removed "code point" and "surrogate code point" in favor of the identical definitions in the Infra Standard.

• Clarified on every range that they are inclusive.

• Added a column to the comment-insertion table to handle a number token appearing next to a "%" delim token.

A Disposition of Comments is available.

12.3. Changes from the 5 November 2013 Last Call Working Draft

• The Serialization section has been rewritten to make only the "round-trip" requirement normative, and move the details of how to achieve it into a note. Some corner cases in these details have been fixed.
• [ENCODING] has been added to the list of normative references. It was already referenced in normative text before, just not listed as such.
• In the algorithm to determine the fallback encoding of a stylesheet, limit the @charset byte sequence to 1024 bytes. This aligns with what HTML does for <meta charset> and makes sure the size of the sequence is bounded. This only makes a difference with leading or trailing whitespace in the encoding label:

  @charset "   (lots of whitespace)   utf-8";

12.4. Changes from the 19 September 2013 Working Draft

• The concept of environment encoding was added. The behavior does not change, but some of the definitions should be moved to the relevant specs.

12.5. Changes from CSS 2.1 and Selectors Level 3

Note: The point of this spec is to match reality; changes from CSS2.1 are nearly always because CSS 2.1 specified something that doesn’t match actual browser behavior, or left something unspecified. If some detail doesn’t match browsers, please let me know as it’s almost certainly unintentional.

Changes in decoding from a byte stream:

• Only detect @charset rules in ASCII-compatible byte patterns.
• Ignore @charset rules that specify an ASCII-incompatible encoding, as that would cause the rule itself to not decode properly.
• Refer to [ENCODING] rather than the IANA registry for character encodings.

Tokenization changes:

• Any U+0000 NULL code point in the CSS source is replaced with U+FFFD REPLACEMENT CHARACTER.
• Any hexadecimal escape sequence such as \0 that evaluates to zero produce U+FFFD REPLACEMENT CHARACTER rather than U+0000 NULL.
• The definition of non-ASCII ident code point was changed to be consistent with HTML’s valid custom elements names.
• Tokenization does not emit COMMENT or BAD_COMMENT tokens anymore. BAD_COMMENT is now considered the same as a normal token (not an error). Serialization is responsible for inserting comments as necessary between tokens that need to be separated, e.g. two consecutive <ident-token>s.
• The <unicode-range-token> was removed, as it was low value and occasionally actively harmful. (u+a { font-weight: bold; } was an invalid selector, for example...)

  Instead, a <urange> production was added, based on token patterns. It is technically looser than what 2.1 allowed (any number of digits and ? characters), but not in any way that should impact its use in practice.

• Apply the EOF error handling rule in the tokenizer and emit normal <string-token> and <url-token> rather than BAD_STRING or BAD_URI on EOF.
• The BAD_URI token (now <bad-url-token>) is "self-contained". In other words, once the tokenizer realizes it’s in a <bad-url-token> rather than a <url-token>, it just seeks forward to look for the closing ), ignoring everything else. This behavior is simpler than treating it like a <function-token> and paying attention to opened blocks and such. Only WebKit exhibits this behavior, but it doesn’t appear that we’ve gotten any compat bugs from it.
• The <comma-token> has been added.
• <number-token>, <percentage-token>, and <dimension-token> have been changed to include the preceding +/- sign as part of their value (rather than as a separate <delim-token> that needs to be manually handled every time the token is mentioned in other specs). The only consequence of this is that comments can no longer be inserted between the sign and the number.
• Scientific notation is supported for numbers/percentages/dimensions to match SVG, per WG resolution.
• Hexadecimal escape for surrogate now emit a replacement character rather than the surrogate. This allows implementations to safely use UTF-16 internally.

Parsing changes:

• Any list of declarations now also accepts at-rules, like @page, per WG resolution. This makes a difference in error handling even if no such at-rules are defined yet: an at-rule, valid or not, ends at a {} block without a <semicolon-token> and lets the next declaration begin.
• The handling of some miscellaneous "special" tokens (like an unmatched <}-token>) showing up in various places in the grammar has been specified with some reasonable behavior shown by at least one browser. Previously, stylesheets with those tokens in those places just didn’t match the stylesheet grammar at all, so their handling was totally undefined. Specifically:
  • [] blocks, () blocks and functions can now contain {} blocks, <at-keyword-token>s or <semicolon-token>s
  • Qualified rule preludes can now contain semicolons
  • Qualified rule and at-rule preludes can now contain <at-keyword-token>s

An+B changes from Selectors Level 3 [SELECT]:

• The An+B microsyntax has now been formally defined in terms of CSS tokens, rather than with a separate tokenizer. This has resulted in minor differences:
  • In some cases, minus signs or digits can be escaped (when they appear as part of the unit of a <dimension-token> or <ident-token>).

Acknowledgments

Thanks for feedback and contributions from Anne van Kesteren, David Baron, Elika J. Etemad (fantasai), Henri Sivonen, Johannes Koch, 呂康豪 (Kang-Hao Lu), Marc O’Morain, Raffaello Giulietti, Simon Pieter, Tyler Karaszewski, and Zack Weinberg.