CSS Mobile Text Size Adjustment Module Level 1

1. Introduction

A common mechanism for displaying Web pages that were designed for large desktop displays on much smaller displays such as those of mobile phones involves allowing the user to pan and zoom around within a view of the Web page drawn as though it were drawn into the width of a typical desktop Web browser display. The ability to pan and zoom the page lets the user both see an overview of the page and zoom in to specific parts to read or interact with them.

One common problem with this type of interaction occurs when the user wants to read a large block of text. It might be that a block of text within this desktop-formatted page might be laid out so that when the user zooms in so that the text is large enough to read, each line of text is wider than the display on the small device. This means the user needs to scroll side to side to read each line of text, which is a serious inconvenience to the user.

One way for software that displays Web pages or other CSS-formatted content on a mobile device is to make some of the text larger so that this problem does not occur. The goal of this enlargement is to make the text big enough so that when the block it is in is scaled to the width of the display, the text is large enough to read. At the same time, this needs to be done with minimal disruption to the overall design of the page.

While implementations of CSS are not required to use this technique, this module describes how implementations of CSS that do use this technique must do so. In other words, while implementations of CSS are not required to implement this module, this module nonetheless places requirements on implementations of this module.

This module describes how this size adjustment works and describes a new CSS property that authors of CSS can use to provide hints to the implementation about which text or other elements should or should not be enlarged.

1.1. Module interactions

This module adds additional features that are not defined in [CSS21]. These features may lead to a different size being computed than would be computed when following [CSS21] alone.

1.2. Value Definitions

This specification follows the CSS property definition conventions from [CSS2] using the value definition syntax from [CSS-VALUES-3]. Value types not defined in this specification are defined in CSS Values & Units [CSS-VALUES-3]. Combination with other CSS modules may expand the definitions of these value types.

In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the CSS-wide keywords as their property value. For readability they have not been repeated explicitly.

2. Default size adjustment

This section defines the default size adjustment rules. These rules are then referenced by the definition of the text-size-adjust property in the following section.

All of the subsections of this section need significant refinement: additional detail, verification that the detail already present is correct, etc.

It’s not clear how much this section should define precise behavior versus how much it should allow future room for innovation and improvement.

2.1. Types of boxes adjusted

The default size adjustment affects text and form controls, whether those form controls contain text (e.g., text inputs, selects) or do not (e.g., radio buttons, checkboxes).

2.2. Conditions that suppress adjustment

A number of conditions suppress the default adjustment because these conditions are associated with layouts for which the user experience would be worsened by size adjustment rather than improved by it. These conditions are:

• when the total amount of text in the block formatting context (see [CSS21]) (excluding text inside descendant block formatting contexts) is approximately smaller than the amount that would require wrapping to more than one or two lines within that context’s width,
• when the objects to be adjusted are inside a block-level or display: inline-block element with a height other than auto (see [CSS21]),
• when the objects to be adjusted are inside a display: inline-block element with a width other than auto (see [CSS21]),
• when the objects to be adjusted have white-space of pre or nowrap (see [CSS21]) or a text-wrap of nowrap (see [CSS-TEXT-4]).

2.3. Calculation of default adjustment

The adjustment performed should be based on preferences (of the renderer or the renderer’s user) indicating the desired minimum readable text size. Given this preference, for each containing block of text to be adjusted, there is a minimum block text size: the preference for the minimum readable text size, times the width of the containing block, divided by the width of the device.

The size adjustment involves multiplication of sizes by a ratio determined by the minimum block text size and the computed value of font-size. This ratio must be at least the first divided by the second; however, in order to maintain differentiations between font sizes, it should often be slightly larger. Define this with more detail/precision.

3. Size adjustment control: the text-size-adjust property

auto

Renderers must use the default size adjustment when displaying on a small device.

none

Renderers must not do size adjustment when displaying on a small device.

<percentage [0,∞]>

When displaying on a small device, renderers must not do size adjustment but instead the computed value of font-size must be multiplied by this percentage.

Note: This means that 'text-size-adjust: 100%;' is equivalent to 'text-size-adjust: none;'.

Negative values are invalid.

"Small device" is not well defined. We should clearly specify when text size adjustment should apply. For example, in practice a large tablet is considered by vendors to be a small device.

4. Acknowledgments

The editors would like to thank: .