csswg-drafts/css-size-adjust/Overview.src.html at 8e16e40e94ae5b7a6b87f04d37e51b5e3a2fc683 · w3c/csswg-drafts · GitHub

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<!DOCTYPE html>
<html lang="en">
<head>
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
  <title>CSS Mobile Text Size Adjustment Module Level 1</title>
  <link rel="stylesheet" type="text/css" href="../default.css">
  <link rel="stylesheet" type="text/css"
        href="http://www.w3.org/StyleSheets/TR/W3C-[STATUS].css">
</head>

<div class="head">
<!--logo-->

<h1>CSS Mobile Text Size Adjustment Module Level 1</h1>

<h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>
<dl>
  <dt>This version:
    <dd><a href="[VERSION]">http://www.w3.org/TR/[YEAR]/ED-css-size-adjust-[CDATE]/</a>

  <dt>Latest version:
    <dd><a href="http://www.w3.org/TR/[SHORTNAME]/">http://www.w3.org/TR/[SHORTNAME]/</a>

  <dt>Editor's draft:
    <dd><a href="http://dev.w3.org/csswg/[SHORTNAME]/">http://dev.w3.org/csswg/[SHORTNAME]/</a>

  <dt>Previous version:
    <dd>none

  <dt>Issue Tracking:</dt>
    <dd>Maintained in document (only editor's draft is current)

    <dt>Feedback:
      <dd><a href="mailto:www-style@w3.org?subject=%5Bcss-size-adjust%5D%20feedback">www-style@w3.org</a>
          with subject line &ldquo;<kbd>[css-size-adjust] <var>&hellip; message topic &hellip;</var></kbd>&rdquo;
          (<a rel="discussion" href="http://lists.w3.org/Archives/Public/www-style/">archives</a>)

  <dt>Editors:
    <dd class="vcard"><a class="fn n url" href="http://dbaron.org/"><abbr class="given-name">L.</abbr>
<span class="additional-name">David</span> <span
class="family-name">Baron</span></a>
     (<span class="org company vcard"><a class="company fn org url" href="http://www.mozilla.org/">Mozilla Corporation</a></span>)
    </dd>
    <dd class="vcard"><a lang="tr" href="http://tantek.com/" class="fn url">Tantek &Ccedil;elik</a>
     (<span class="org company vcard"><a class="company fn org url" href="http://www.mozilla.org/">Mozilla Corporation</a></span>)
    </dd>
</dl>

<!--copyright-->

<hr title="Separator for header">
</div>

<h2 class="no-num no-toc" id="abstract">Abstract</h2>

  <p>CSS is a language for describing the rendering of structured documents
  (such as HTML and XML) on screen, on paper, in speech, etc.  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.</p>

<h2 class="no-num no-toc" id="status">Status of this document</h2>

<!--status-->

<p>The following features are at risk:</p>
<ul>
  <li>the &lt;percentage&gt; value for 'text-size-adjust'</li>
  <li>the reference to [[!CSS3TEXT]]&apos;s 'text-wrap' property</li>
</ul>

<h2 class="no-num no-toc" id="contents">Table of contents</h2>

<!--toc-->

<h2 id="intro">Introduction</h2>

  <p>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.</p>

  <p>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 <em>each line of text</em>,
  which is a serious inconvenience to the user.</p>

  <p>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 
8000
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.</p>

  <p>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.</p>

  <p>
  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.
  </p>

<h3 id="placement">Module interactions</h3>

  <p>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.</p>

<h3 id="values">Values</h3>

  <p>This specification follows the
  <a href="http://www.w3.org/TR/CSS21/about.html#property-defs">CSS property
  definition conventions</a> from [[!CSS21]]. Value types not defined in
  this specification are defined in CSS Level 2 Revision 1 [[!CSS21]].
  Other CSS modules may expand the definitions of these value types: for
  example [[CSS3COLOR]], when combined with this module, expands the
  definition of the &lt;color&gt; value type as used in this specification.</p>

  <p>In addition to the property-specific values listed in their definitions,
  all properties defined in this specification also accept the
  <a href="http://www.w3.org/TR/CSS21/cascade.html#value-def-inherit">inherit</a>
  keyword as their property value. For readability it has not been repeated
  explicitly.

<h2 id="default-adjustment">Default size adjustment</h2>

<p>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.</p>

<p class="issue">All of the subsections of this section need significant
refinement:  additional detail, verification that the detail already
present is correct, etc.</p>

<p class="issue">It's not clear how much this section should define
precise behavior versus how much it should allow future room for
innovation and improvement.</p>

<h3 id="default-adjustment-types">Types of boxes adjusted</h3>

<p>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).</p>

<h3 id="default-adjustment-conditions">Conditions that suppress adjustment</h3>

<p>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:</p>

<ul>
  <li>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,</li>

  <li>when the objects to be adjusted are inside a block-level or
  ''display: inline-block'' element with a 'height' other than
  ''auto'' (see [[!CSS21]]),</li>

  <li>when the objects to be adjusted are inside a ''display:
  inline-block'' element with a 'width' other than ''auto'' (see
  [[!CSS21]]),</li>

  <li>when the objects to be adjusted have 'white-space' of ''pre'' or
  ''nowrap'' (see [[!CSS21]]) or a 'text-wrap' of ''none'' (see
  [[!CSS3TEXT]]).</li>

</ul>

<h3 id="default-adjustment-calculation">Calculation of default adjustment</h3>

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

<p>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.  <span class="issue">Define
this with more detail/precision.</span></p>


<h2 id="adjustment-control">Size adjustment control: the 'text-size-adjust' property</h2>

  <table class=propdef>
    <tr>
      <th>Name:
      <td><dfn>text-size-adjust</dfn>
    <tr>
      <th><a href="#values">Value</a>:
      <td>auto | none | &lt;percentage&gt;
    <tr>
      <th>Initial:
      <td>auto
    <tr>
      <th>Applies to:
    <tr>
      <th>Inherited:
      <td>yes
    <tr>
      <th>Percentages:
      <td>see below
    <tr>
      <th>Media:
      <td>visual
    <tr>
      <th>Computed&nbsp;value:
      <td>as specified
    <tr>
      <th>Animatable:
      <td>as percentage
    <tr>
      <th>Canonical order:
      <td>N/A
  </table>

  <dl>
    <dt><dfn>''auto''</dfn></dt>
      <dd>Renderers must use the <a href="#default-adjustment">default size adjustment</a> when displaying on a small device.</dd>
    <dt><dfn>''none''</dfn></dt>
      <dd>Renderers must not do size adjustment when displaying on a small device.</dd>
    <dt><dfn>&lt;percentage&gt;</dfn></dt>
      <dd><span class="issue">Need to define what percentages actually mean.  Are they a minimum or a set value?  What exactly are they relative to?</span></dd>
  </dl>

<h2 id="conformance">Conformance</h2>

<h3 id="conventions">Document conventions</h3>

  <p>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.

  <p>All of the text of this specification is normative except sections
  explicitly marked as non-normative, examples, and notes. [[!RFC2119]]</p>

  <p>Examples in this specification are introduced with the words “for example”
  or are set apart from the normative text with <code>class="example"</code>,
  like this:

  <div class="example">
    <p>This is an example of an informative example.</p>
  </div>

  <p>Informative notes begin with the word “Note” and are set apart from the
  normative text with <code>class="note"</code>, like this:

  <p class="note">Note, this is an informative note.</p>

<h3 id="conformance-classes">Conformance classes</h3>

  <p>Conformance to CSS Mobile Text Size Adjustment Module
  is defined for three conformance classes:
  <dl>
    <dt><dfn title="style sheet!!as conformance class">style sheet</dfn>
      <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
      style sheet</a>.
    <dt><dfn>renderer</dfn></dt>
      <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
      that interprets the semantics of a style sheet and renders
      documents that use them.
    <dt><dfn id="authoring-tool">authoring tool</dfn></dt>
      <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
      that writes a style sheet.
  </dl>

  <p>A style sheet is conformant to CSS Mobile Text Size Adjustment Module
  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.

  <p>A renderer is conformant to CSS Mobile Text Size Adjustment Module
  if, in addition to interpreting the style sheet as defined by the
  appropriate specifications, it supports all the features defined
  by CSS Mobile Text Size Adjustment Module 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.)

  <p>An authoring tool is conformant to CSS Mobile Text Size Adjustment Module
  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.

<h3 id="partial">Partial implementations</h3>

  <p>So that authors can exploit the forward-compatible parsing rules to
  assign fallback values, CSS renderers <strong>must</strong>
  treat as invalid (and <a href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignore
  as appropriate</a>) any at-rules, properties, property values, keywords,
  and other syntactic constructs for which they have no usable level of
  support. In particular, user agents <strong>must not</strong> 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
  be ignored.</p>

<h3 id="experimental">Experimental implementations</h3>

  <p>To avoid clashes with future CSS features, the CSS2.1 specification
  reserves a <a href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
  syntax</a> for proprietary and experimental extensions to CSS.

  <p>Prior to a specification reaching the Candidate Recommendation stage
  in the W3C process, all implementations of a CSS feature are considered
  experimental. The CSS Working Group recommends that implementations
  use a vendor-prefixed syntax for such features, including those in
  W3C Working Drafts. This avoids incompatibilities with future changes
  in the draft.
  </p>

<h3 id="testing">Non-experimental implementations</h3>

  <p>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.

  <p>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
  Working Group.

  <p>Further information on submitting testcases and implementation reports
  can be found from on the CSS Working Group's website at
  <a href="http://www.w3.org/Style/CSS/Test/">http://www.w3.org/Style/CSS/Test/</a>.
  Questions should be directed to the
  <a href="http://lists.w3.org/Archives/Public/public-css-testsuite">public-css-testsuite@w3.org</a>
  mailing list.

<h3 id="cr-exit-criteria">CR exit criteria</h3>

  <p>
  For this specification to be advanced to Proposed Recommendation,
  there must be at least two independent, interoperable implementations
  of each feature. Each feature may be implemented by a different set of
  products, there is no requirement that all features be implemented by
  a single product. For the purposes of this criterion, we define the
  following terms:

  <dl>
    <dt>independent <dd>each implementation must be developed by a
    different party and cannot share, reuse, or derive from code
    used by another qualifying implementation. Sections of code that
    have no bearing on the implementation of this specification are
    exempt from this requirement.

    <dt>interoperable <dd>passing the respective test case(s) in the
    official CSS test suite, or, if the implementation is not a Web
    browser, an equivalent test. Every relevant test in the test
    suite should have an equivalent test created if such a user
    agent (UA) is to be used to claim interoperability. In addition
    if such a UA is to be used to claim interoperability, then there
    must one or more additional UAs which can also pass those
    equivalent tests in the same way for the purpose of
    interoperability. The equivalent tests must be made publicly
    available for the purposes of peer review.

    <dt>implementation <dd>a user agent which:

    <ol class=inline>
      <li>implements the specification.

      <li>is available to the general public. The implementation may
      be a shipping product or other publicly available version
      (i.e., beta version, preview release, or “nightly build”).
      Non-shipping product releases must have implemented the
      feature(s) for a period of at least one month in order to
      demonstrate stability.

      <li>is not experimental (i.e., a version specifically designed
      to pass the test suite and is not intended for normal usage
      going forward).
    </ol>
  </dl>

  <p>The specification will remain Candidate Recommendation for at least
  six months.

<h2 class=no-num id="acknowledgments">Acknowledgments</h2>

  <p>The editors would like to thank:
  .</p>

<h2 class=no-num id="references">References</h2>

<h3 class="no-num" id="normative-references">Normative references</h3>
<!--normative-->

<h3 class="no-num" id="other-references">Other references</h3>
<!--informative-->

<h2 class="no-num" id="index">Index</h2>
<!--index-->

<h2 class="no-num" id="property-index">Property index</h2>
<!-- properties -->

</body>
</html>
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