This module contains features of CSS relating to one possible mechanism for adapting pages designed for desktop computer displays for display on smaller screens such as those of mobile phones. This mechanism involves displaying a scaled down display of the Web page and allowing the user to pan and zoom within that display, but within that scaled down display making certain text and similar elements larger than specified by the page author in order to ensure that when a block of wrapped text is zoomed to the width of the device (so it can be read without side-to-side scrolling for each line), the text is large enough to be readable.
CSS is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, etc.
Status of this document
This is a public copy of the editors’ draft.
It is provided for discussion only and may change at any moment.
Its publication here does not imply endorsement of its contents by W3C.
Don’t cite this document other than as work in progress.
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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
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.
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
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
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.
Renderers must not do size adjustment when displaying on a small device.
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.
The editors would like to thank:
Conformance requirements are expressed with a combination of
descriptive assertions and RFC 2119 terminology. The key words “MUST”,
“MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
“RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
document are to be interpreted as described in RFC 2119.
However, for readability, these words do not appear in all uppercase
letters in this specification.
All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. [RFC2119]
Examples in this specification are introduced with the words “for example”
or are set apart from the normative text with class="example",
This is an example of an informative example.
Informative notes begin with the word “Note” and are set apart from the
normative text with class="note", like this:
Note, this is an informative note.
Advisements are normative sections styled to evoke special attention and are
set apart from other normative text with <strong class="advisement">, like
this: UAs MUST provide an accessible alternative.
Conformance to this specification
is defined for three conformance classes:
A style sheet is conformant to this specification
if all of its statements that use syntax defined in this module are valid
according to the generic CSS grammar and the individual grammars of each
feature defined in this module.
A renderer is conformant to this specification
if, in addition to interpreting the style sheet as defined by the
appropriate specifications, it supports all the features defined
by this specification by parsing them correctly
and rendering the document accordingly. However, the inability of a
UA to correctly render a document due to limitations of the device
does not make the UA non-conformant. (For example, a UA is not
required to render color on a monochrome monitor.)
An authoring tool is conformant to this specification
if it writes style sheets that are syntactically correct according to the
generic CSS grammar and the individual grammars of each feature in
this module, and meet all other conformance requirements of style sheets
as described in this module.
So that authors can exploit the forward-compatible parsing rules to
assign fallback values, CSS renderers must treat as invalid (and ignore
as appropriate) any at-rules, properties, property values, keywords,
and other syntactic constructs for which they have no usable level of
support. In particular, user agents must not selectively
ignore unsupported component values and honor supported values in a single
multi-value property declaration: if any value is considered invalid
(as unsupported values must be), CSS requires that the entire declaration
Implementations of Unstable and Proprietary Features
Once a specification reaches the Candidate Recommendation stage,
non-experimental implementations are possible, and implementors should
release an unprefixed implementation of any CR-level feature they
can demonstrate to be correctly implemented according to spec.
To establish and maintain the interoperability of CSS across
implementations, the CSS Working Group requests that non-experimental
CSS renderers submit an implementation report (and, if necessary, the
testcases used for that implementation report) to the W3C before
releasing an unprefixed implementation of any CSS features. Testcases
submitted to W3C are subject to review and correction by the CSS