CSS Font Loading Module Level 3

1. Introduction

CSS allows authors to load custom fonts from the web via the @font-face rule. While this is easy to use when authoring a stylesheet, it’s much more difficult to use dynamically via scripting.

Further, CSS allows the user agent to choose when to actually load a font; if a font face isn’t currently used by anything on a page, most user agents will not download its associated file. This means that later use of the font face will incur a delay as the user agent finally notices a usage and begins downloading and parsing the font file.

This specification defines a scripting interface to font faces in CSS, allowing font faces to be easily created (via the FontFace interface) and loaded from script (via document.fonts). It also provides methods to track the loading status of an individual font, or of all the fonts on an entire page.

Several things in this spec use normal ES objects to define behavior, such as various things using Promises internally, and FontFaceSet using a Set internally. I believe the intention here is that these objects (and their prototype chains) are pristine, unaffected by anything the author has done. Is this a good intention? If so, how should I indicate this in the spec?

1.1. Values

This specification uses Promises, which are defined in ECMAScript 6. HTML5Rocks has some good tutorial material introducing Promises.

1.2. Task Sources

Whenever this specification queues a task, it queues it onto the "font loading" task source.

2. The FontFace Interface

The FontFace interface represents a single usable font face. CSS @font-face rules implicitly define FontFace objects, or they can be constructed manually from a url or binary data.

typedef (ArrayBuffer or ArrayBufferView) BinaryData;

dictionary FontFaceDescriptors {
  CSSOMString style = "normal";
  CSSOMString weight = "normal";
  CSSOMString stretch = "normal";
  CSSOMString unicodeRange = "U+0-10FFFF";
  CSSOMString variant = "normal";
  CSSOMString featureSettings = "normal";
  CSSOMString variationSettings = "normal";
  CSSOMString display = "auto";
  CSSOMString ascentOverride = "normal";
  CSSOMString descentOverride = "normal";
  CSSOMString lineGapOverride = "normal";
};

enum FontFaceLoadStatus { "unloaded", "loading", "loaded", "error" };

[Exposed=(Window,Worker)]
interface FontFace {
  constructor(CSSOMString family, (CSSOMString or BinaryData) source,
                optional FontFaceDescriptors descriptors = {});
  attribute CSSOMString family;
  attribute CSSOMString style;
  attribute CSSOMString weight;
  attribute CSSOMString stretch;
  attribute CSSOMString unicodeRange;
  attribute CSSOMString variant;
  attribute CSSOMString featureSettings;
  attribute CSSOMString variationSettings;
  attribute CSSOMString display;
  attribute CSSOMString ascentOverride;
  attribute CSSOMString descentOverride;
  attribute CSSOMString lineGapOverride;

  readonly attribute FontFaceLoadStatus status;

  Promise<FontFace> load();
  readonly attribute Promise<FontFace> loaded;
};

Clarify all mentions of "the document" to be clear about which document is being referenced, since objects can move between documents.

All FontFace objects contain an internal [[FontStatusPromise]] slot, which tracks the status of the font. It starts out pending, and fulfills or rejects when the font is successfully loaded and parsed, or hits an error.

All FontFace objects also contain internal [[Urls]] and [[Data]] slots, of which one is null and the other is not null (the non-null one is set by the constructor, based on which data is passed in).

2.1. The Constructor

A FontFace can be constructed either from a URL pointing to a font face file, or from an ArrayBuffer (or ArrayBufferView) containing the binary representation of a font face.

When the FontFace(family, source, descriptors) method is called, execute these steps:

1. Let font face be a fresh FontFace object. Set font face’s status attribute to "unloaded", Set its internal [[FontStatusPromise]] slot to a fresh pending Promise object.

   Parse the family argument, and the members of the descriptors argument, according to the grammars of the corresponding descriptors of the CSS @font-face rule. If the source argument is a CSSOMString, parse it according to the grammar of the CSS src descriptor of the @font-face rule. If any of them fail to parse correctly, reject font face’s [[FontStatusPromise]] with a DOMException named "SyntaxError", set font face’s corresponding attributes to the empty string, and set font face’s status attribute to "error". Otherwise, set font face’s corresponding attributes to the serialization of the parsed values.

   Note: Note that this means that passing a naked url as the source argument, like "http://example.com/myFont.woff", won’t work - it needs to be at least wrapped in a url() function, like "url(http://example.com/myFont.woff)". In return for this inconvenience, you get to specify multiple fallbacks, specify the type of font each fallback is, and refer to local fonts easily.

   Need to define the base url, so relative urls can resolve. Should it be the url of the document? Is that correct for workers too, or should they use their worker url? Is that always defined?

   Return font face. If font face’s status is "error", terminate this algorithm; otherwise, complete the rest of these steps asynchronously.

2. If the source argument was a CSSOMString, set font face’s internal [[Urls]] slot to the string.

   If the source argument was a BinaryData, set font face’s internal [[Data]] slot to the passed argument.

3. If font face’s [[Data]] slot is not null, queue a task to run the following steps synchronously:

   1. Set font face’s status attribute to "loading".

   2. For each FontFaceSet font face is in:

      1. If the FontFaceSet’s [[LoadingFonts]] list is empty, switch the FontFaceSet to loading.

      2. Append font face to the FontFaceSet’s [[LoadingFonts]] list.

   Asynchronously, attempt to parse the data in it as a font. When this is completed, successfully or not, queue a task to run the following steps synchronously:

   1. If the load was successful, font face now represents the parsed font; fulfill font face’s [[FontStatusPromise]] with font face, and set its status attribute to "loaded".

      For each FontFaceSet font face is in:

      1. Add font face to the FontFaceSet’s [[LoadedFonts]] list.

      2. Remove font face from the FontFaceSet’s [[LoadingFonts]] list. If font was the last item in that list (and so the list is now empty), switch the FontFaceSet to loaded.

   2. Otherwise, reject font face’s [[FontStatusPromise]] with a DOMException named "SyntaxError" and set font face’s status attribute to "error".

      For each FontFaceSet font face is in:

      1. Add font face to the FontFaceSet’s [[FailedFonts]] list.

      2. Remove font face from the FontFaceSet’s [[LoadingFonts]] list. If font was the last item in that list (and so the list is now empty), switch the FontFaceSet to loaded.

Note: Newly constructed FontFace objects are not automatically added to the FontFaceSet associated with a document or a context for a worker thread. This means that while newly constructed fonts can be preloaded, they cannot actually be used until they are explicitly added to a FontFaceSet. See the following section for a more complete description of FontFaceSet.

2.2. The load() method

The load() method of FontFace forces a url-based font face to request its font data and load. For fonts constructed from binary data, or fonts that are already loading or loaded, it does nothing.

When the load() method is called, execute these steps:

1. Let font face be the FontFace object on which this method was called.
2. If font face’s [[Urls]] slot is null, or its status attribute is anything other than "unloaded", return font face’s [[FontStatusPromise]] and abort these steps.
3. Otherwise, set font face’s status attribute to "loading", return font face’s [[FontStatusPromise]], and continue executing the rest of this algorithm asynchronously.
4. Using the value of font face’s [[Urls]] slot, attempt to load a font as defined in [CSS-FONTS-3], as if it was the value of a @font-face rule’s src descriptor.
5. When the load operation completes, successfully or not, queue a task to run the following steps synchronously:
   1. If the attempt to load fails, reject font face’s [[FontStatusPromise]] with a DOMException whose name is "NetworkError" and set font face’s status attribute to "error".

      For each FontFaceSet font face is in:

      1. Add font face to the FontFaceSet’s [[FailedFonts]] list.

      2. Remove font face from the FontFaceSet’s [[LoadingFonts]] list. If font was the last item in that list (and so the list is now empty), switch the FontFaceSet to loaded.

   2. Otherwise, font face now represents the loaded font; fulfill font face’s [[FontStatusPromise]] with font face and set font face’s status attribute to "loaded".

      For each FontFaceSet font face is in:

      1. Add font face to the FontFaceSet’s [[LoadedFonts]] list.

      2. Remove font face from the FontFaceSet’s [[LoadingFonts]] list. If font was the last item in that list (and so the list is now empty), switch the FontFaceSet to loaded.

User agents can initiate font loads on their own, whenever they determine that a given font face is necessary to render something on the page. When this happens, they must act as if they had called the corresponding FontFace’s load() method described here.

Note: Some UAs utilize a "font cache" which avoids having to download the same font multiple times on a page or on multiple pages within the same origin. Multiple FontFace objects can be mapped to the same entry in the font cache, which means that a FontFace object might start loading unexpectedly, even if it’s not in a FontFaceSet, because some other FontFace object pointing to the same font data (perhaps on a different page entirely!) has been loaded.

2.3. Interaction with CSS’s @font-face Rule

A CSS @font-face rule automatically defines a corresponding FontFace object, which is automatically placed in the document’s font source when the rule is parsed. This FontFace object is CSS-connected.

The FontFace object corresponding to a @font-face rule has its family, style, weight, stretch, unicodeRange, variant, and featureSettings attributes set to the same value as the corresponding descriptors in the @font-face rule. There is a two-way connection between the two: any change made to a @font-face descriptor is immediately reflected in the corresponding FontFace attribute, and vice versa.

When a FontFace is transferred between documents, it’s no longer CSS-connected.

The internal [[Urls]] slot of the FontFace object is set to the value of the @font-face rule’s src descriptor, and reflects any changes made to the src descriptor.

Otherwise, a FontFace object created by a CSS @font-face rule is identical to one created manually.

If a @font-face rule is removed from the document, its corresponding FontFace object is no longer CSS-connected. The connection is not restorable by any means (but adding the @font-face back to the stylesheet will create a brand new FontFace object which is CSS-connected).

If a @font-face rule has its src descriptor changed to a new value, the original connected FontFace object must stop being CSS-connected. A new FontFace reflecting its new src must be created and CSS-connected to the @font-face. (This will also remove the old and add the new FontFace objects from any font sources they appear in.)

2.4. Discovery of information about a font

A FontFace object includes a variety of read-only information about the contents of the font file.

[Exposed=(Window,Worker)]
interface FontFaceFeatures {
  /* The CSSWG is still discussing what goes in here */
};

[Exposed=(Window,Worker)]
interface FontFaceVariationAxis {
  readonly attribute DOMString name;
  readonly attribute DOMString axisTag;
  readonly attribute double minimumValue;
  readonly attribute double maximumValue;
  readonly attribute double defaultValue;
};

[Exposed=(Window,Worker)]
interface FontFaceVariations {
  readonly setlike<FontFaceVariationAxis>;
};

[Exposed=(Window,Worker)]
interface FontFacePalette {
  iterable<DOMString>;
  readonly attribute unsigned long length;
  getter DOMString (unsigned long index);
  readonly attribute boolean usableWithLightBackground;
  readonly attribute boolean usableWithDarkBackground;
};

[Exposed=(Window,Worker)]
interface FontFacePalettes {
  iterable<FontFacePalette>;
  readonly attribute unsigned long length;
  getter FontFacePalette (unsigned long index);
};

partial interface FontFace {
  readonly attribute FontFaceFeatures features;
  readonly attribute FontFaceVariations variations;
  readonly attribute FontFacePalettes palettes;
};

Note: This read-only data is intended to help authors know which values are accepted by font-feature-settings, font-variation-settings, and @font-palette-values.

3. The FontFaceSet Interface

dictionary FontFaceSetLoadEventInit : EventInit {
  sequence<FontFace> fontfaces = [];
};

[Exposed=(Window,Worker)]
interface FontFaceSetLoadEvent : Event {
  constructor(CSSOMString type, optional FontFaceSetLoadEventInit eventInitDict = {});
  [SameObject] readonly attribute FrozenArray<FontFace> fontfaces;
};

enum FontFaceSetLoadStatus { "loading", "loaded" };

[Exposed=(Window,Worker)]
interface FontFaceSet : EventTarget {
  constructor(sequence<FontFace> initialFaces);

  setlike<FontFace>;
  FontFaceSet add(FontFace font);
  boolean delete(FontFace font);
  undefined clear();

  // events for when loading state changes
  attribute EventHandler onloading;
  attribute EventHandler onloadingdone;
  attribute EventHandler onloadingerror;

  // check and start loads if appropriate
  // and fulfill promise when all loads complete
  Promise<sequence<FontFace>> load(CSSOMString font, optional CSSOMString text = " ");

  // return whether all fonts in the fontlist are loaded
  // (does not initiate load if not available)
  boolean check(CSSOMString font, optional CSSOMString text = " ");

  // async notification that font loading and layout operations are done
  readonly attribute Promise<FontFaceSet> ready;

  // loading state, "loading" while one or more fonts loading, "loaded" otherwise
  readonly attribute FontFaceSetLoadStatus status;
};

FontFaceSet objects also have internal [[LoadingFonts]], [[LoadedFonts]], and [[FailedFonts]] slots, all of which are initialized to empty lists, and a [[ReadyPromise]] slot, which is initialized to a fresh pending Promise.

Because font families are loaded only when they are used, content sometimes needs to understand when the loading of fonts occurs. Authors can use the events and methods defined here to allow greater control over actions that are dependent upon the availability of specific fonts.

A FontFaceSet is pending on the environment if any of the following are true:

• the document is still loading

• the document has pending stylesheet requests

• the document has pending layout operations which might cause the user agent to request a font, or which depend on recently-loaded fonts

Note: The idea is that once a FontFaceSet stops being pending on the environment, as long as nothing further changes the document, an author can depend on sizes/positions of things being "correct" when measured. If the above conditions do not fully capture this guarantee, they need to be amended to do so.

3.1. Events

Font load events make it easy to respond to the font-loading behavior of the entire document, rather than having to listen to each font specifically. The loading event fires when the document begins loading fonts, while the loadingdone and loadingerror events fire when the document is done loading fonts, containing the fonts that successfully loaded or failed to load, respectively.

The following are the event handlers (and their corresponding event handler event types) that must be supported by FontFaceSet objects as IDL attributes:

To fire a font load event named e at a FontFaceSet target with optional font faces means to fire a simple event named e using the FontFaceSetLoadEvent interface that also meets these conditions:

1. The fontfaces attribute is initialized to the result of filtering font faces to only contain FontFace objects contained in target.

When asked to switch the FontFaceSet to loading for a given FontFaceSet, the user agent must run the following steps:

1. Let font face set be the given FontFaceSet.
2. Set the status attribute of font face set to "loading".
3. If font face set’s [[ReadyPromise]] slot currently holds a fulfilled promise, replace it with a fresh pending promise.
4. Queue a task to fire a font load event named loading at font face set.

When asked to switch the FontFaceSet to loaded for a given FontFaceSet, the user agent must run the following steps:

1. Let font face set be the given FontFaceSet.

2. If font face set is pending on the environment, mark it as stuck on the environment, and exit this algorithm.

3. Set font face set’s status attribute to "loaded".

4. Fulfill font face set’s [[ReadyPromise]] attribute’s value with font face set.

5. Queue a task to perform the following steps synchronously:

   1. Let loaded fonts be the (possibly empty) contents of font face set’s [[LoadedFonts]] slot.

   2. Let failed fonts be the (possibly empty) contents of font face set’s [[FailedFonts]] slot.

   3. Reset the [[LoadedFonts]] and [[FailedFonts]] slots to empty lists.

   4. Fire a font load event named loadingdone at font face set with loaded fonts.

   5. If font face set’s failed fonts is non-empty, fire a font load event named loadingerror at font face set with failed fonts.

Whenever a FontFaceSet goes from pending on the environment to not pending on the environment, the user agent must run the following steps:

1. If the FontFaceSet is stuck on the environment and its [[LoadingFonts]] list is empty, switch the FontFaceSet to loaded.

2. If the FontFaceSet is stuck on the environment, unmark it as such.

If asked to find the matching font faces from a FontFaceSet source, for a given font string font optionally some sample text text, and optionally an allow system fonts flag, run the following steps:

1. Parse font using the CSS value syntax of the font property. If a syntax error occurs, return a syntax error.

   If the parsed value is a CSS-wide keyword, return a syntax error.

   Absolutize all relative lengths against the initial values of the corresponding properties. (For example, a relative font weight like bolder is evaluated against the initial value normal.)

2. If text was not explicitly provided, let it be a string containing a single space character (U+0020 SPACE).
3. Let font family list be the list of font families parsed from font, and font style be the other font style attributes parsed from font.
4. Let available font faces be the available font faces within source. If the allow system fonts flag is specified, add all system fonts to available font faces.
5. Let matched font faces initially be an empty list.
6. For each family in font family list, use the font matching rules to select the font faces from available font faces that match the font style, and add them to matched font faces. The use of the unicodeRange attribute means that this may be more than just a single font face.
7. If matched font faces is empty, set the found faces flag to false. Otherwise, set it to true.
8. For each font face in matched font faces, if its defined unicode-range does not include the codepoint of at least one character in text, remove it from the list.

   Note: Therefore, if text is the empty string, every font will be removed.

9. Return matched font faces and the found faces flag.

3.2. The load() method

The load() method of FontFaceSet will determine whether all fonts in the given font list have been loaded and are available. If any fonts are downloadable fonts and have not already been loaded, the user agent will initiate the load of each of these fonts. It returns a Promise, which is fulfilled when all of the fonts are loaded and ready to be used, or rejected if any font failed to load properly.

When the load( font, text ) method is called, execute these steps:

1. Let font face set be the FontFaceSet object this method was called on. Let promise be a newly-created promise object.
2. Return promise. Complete the rest of these steps asynchronously.
3. Find the matching font faces from font face set using the font and text arguments passed to the function, and let font face list be the return value (ignoring the found faces flag). If a syntax error was returned, reject promise with a SyntaxError exception and terminate these steps.
4. Queue a task to run the following steps synchronously:
   1. For all of the font faces in the font face list, call their load() method.
   2. Resolve promise with the result of waiting for all of the [[FontStatusPromise]]s of each font face in the font face list, in order.

3.3. The check() method

The check() method of FontFaceSet will determine whether you can "safely" render some provided text with a particular font list, such that it won’t cause a "font swap" later. If the given text/font combo will render without attempting to use any unloaded or currently-loading fonts, this method will return true; otherwise, it returns false.

When the check( font, text) method is called, execute these steps:

1. Let font face set be the FontFaceSet object this method was called on.
2. Find the matching font faces from font face set using the font and text arguments passed to the function, and including system fonts, and let font face list be the returned list of font faces, and found faces be the returned found faces flag. If a syntax error was returned, throw a SyntaxError exception and terminate these steps.
3. If font face list is empty, or all fonts in the font face list either have a status attribute of "loaded" or are system fonts, return true. Otherwise, return false.

3.4. The ready attribute

Because the number of fonts loaded depends on the how many fonts are used for a given piece of text, in some cases whether fonts need to be loaded or not may not be known. The ready attribute contains a Promise which is resolved when the document is done loading fonts, which provides a way for authors to avoid having to keep track of which fonts have or haven’t been loaded before examining content which may be affected by loading fonts.

Note: Authors should note that a given ready promise is only fulfilled once, but further fonts may be loaded after it fulfills. This is similar to listening for a loadingdone event to fire, but the callbacks passed to the ready promise will always get called, even when no font loads occur because the fonts in question are already loaded. It’s a simple, easy way to synchronize code to font loads without the need to keep track of what fonts are needed and precisely when they load.

Note: Note that the user agent may need to iterate over multiple font loads before the ready promise is fulfilled. This can occur with font fallback situations, where one font in the fontlist is loaded but doesn’t contain a particular glyph and other fonts in the fontlist need to be loaded. The ready promise is only fulfilled after layout operations complete and no additional font loads are necessary.

Note: Note that the Promise returned by this ready attribute is only ever fulfilled, never rejected, unlike the Promise returned by the FontFace load() method.

3.5. Interaction with CSS Font Loading and Matching

When the font matching algorithm in [CSS-FONTS-3] is run automatically by the user-agent, the set of font faces it matches over must be precisely the set of fonts in the font source for the document, plus any local font faces.

When a user-agent needs to load a font face, it must do so by calling the load() method of the corresponding FontFace object.

(This means it must run the same algorithm, not literally call the value currently stored in the load property of the object.)

document.styleSheets[0].insertRule(
  "@font-face { font-family: newfont; src: url(newfont.woff); }", 0);
document.body.style.fontFamily = "newfont, serif";

var f = new FontFace("newfont", "url(newfont.woff)");
document.fonts.add(f);
document.body.style.fontFamily = "newfont, serif";

var f = new FontFace("newfont", "url(newtest.woff)", {});

/* new {{FontFace}} not added to {{FontFaceSet}},
   so the 'font-family' property can’t see it,
   and serif will be used instead */
document.body.style.fontFamily = "newfont, serif";

var f = new FontFace("newfont", "url(newfont.woff)", {});
f.load().then(function (loadedFace) {
  document.fonts.add(loadedFace);
  document.body.style.fontFamily = "newfont, serif";
});

4. The FontFaceSource Mixin

interface mixin FontFaceSource {
  readonly attribute FontFaceSet fonts;
};

Document includes FontFaceSource;
WorkerGlobalScope includes FontFaceSource;

Any document, workers, or other context which can use fonts in some manner must include the FontFaceSource mixin. The value of the context’s fonts attribute is its font source, which provides all of the fonts used in font-related operations, unless defined otherwise. Operations referring to “the font source” must be interpreted as referring to the font source of the relevant context in which the operation is taking place.

For any font-related operation that takes place within one of these contexts, the FontFace objects within the font source are its available font faces.

4.1. Worker FontFaceSources

Within a Worker document, the font source is initially empty.

Note: FontFace objects can be constructed and added to it as normal, which affects CSS font-matching within the worker (such as, for example, drawing text into a OffscreenCanvas).

4.2. Interaction with CSS’s @font-face Rule

The set entries for a document’s font source must be initially populated with all the CSS-connected FontFace objects from all of the CSS @font-face rules in the document or shadow root CSS style sheets, in document order. As @font-face rules are added or removed from a stylesheet, or stylesheets containing @font-face rules are added or removed, the corresponding CSS-connected FontFace objects must be added or removed from the document’s font source, and maintain this ordering.

Any manually-added FontFace objects must be ordered after the CSS-connected ones.

When a FontFaceSet object’s add() method is called with a CSS-connected FontFace object, if the object is already in the set, the operation must be a no-op; otherwise, the operation must do nothing, and throw an InvalidModificationError.

When a FontFaceSet object’s delete() method is called with a CSS-connected FontFace object, the operation must be a no-op, and return false.

Note: Authors can still maintain references to a removed FontFace, even if it’s been automatically removed from a font source. As specified in § 2.3 Interaction with CSS’s @font-face Rule, though, the FontFace is no longer CSS-connected at that point.

Note: It is expected that a future version of this specification will define ways of interacting with and querying local fonts as well.

5. API Examples

document.fonts.ready.then(function() {
  var content = document.getElementById("content");
  content.style.visibility = "visible";
});

function drawStuff() {
  var ctx = document.getElementById("c").getContext("2d");

  ctx.fillStyle = "red";
  ctx.font = "50px MyDownloadableFont";
  ctx.fillText("Hello!", 100, 100);
}

document.fonts.load("50px MyDownloadableFont")
              .then(drawStuff, handleError);

function measureTextElements() {
  // contents can now be measured using the metrics of
  // the downloadable font(s)
}

function doEditing() {
  // content/layout operations that may cause additional font loads
  document.fonts.ready.then(measureTextElements);
}

<style>
@font-face {
  font-family: latin-serif;
  src: url(latinserif.woff) format("woff"); /* contains no kanji/kana */
}
@font-face {
  font-family: jpn-mincho;
  src: url(mincho.woff) format("woff");
}
@font-face {
  font-family: unused;
  src: url(unused.woff);
}

body { font-family: latin-serif, jpn-mincho; }
</style>
<p>納豆はいかがでしょうか

Changes

Changes from the May 2014 CSS Font Loading Last Call Working Draft:

1. Corrected the async algorithms to use "queue a task" language, to ensure that side-effect timing is well-defined.

2. Switched fontfaces to be a FrozenArray, to match with proper IDL practice.

3. Added variationSettings and display, to sync with @font-face.

Acknowledgments

Several members of the Google Fonts team provided helpful feedback on font load events, as did Boris Zbarsky, Jonas Sicking and ms2ger.

Privacy & Security Considerations

The FontFaceSet object leaks information about the user’s installed fonts, but in the exact same way as the existing @font-face rule; no new information is leaked, or in any appreciable easier manner.

This specification introduces no new security considerations.