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<!doctype html><html lang="en">
<head>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
<title>CSS Mobile Text Size Adjustment Module Level 1</title>
<meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
<meta content="revising" name="csswg-work-status">
<meta content="ED" name="w3c-status">
<link href="../default.css" rel="stylesheet" type="text/css">
<link href="../csslogo.ico" rel="shortcut icon" type="image/x-icon">
<style>
body {
background-image: url("https://www.w3.org/StyleSheets/TR/logo-ED");
};
</style>
<meta content="Bikeshed 1.0.0" name="generator">
<style>
.dfn-panel {
display: inline-block;
position: absolute;
z-index: 35;
height: auto;
width: -webkit-fit-content;
max-width: 300px;
max-height: 500px;
overflow: auto;
padding: 0.5em 0.75em;
font: small Helvetica Neue, sans-serif, Droid Sans Fallback;
background: #DDDDDD;
color: black;
border: outset 0.2em;
}
.dfn-panel:not(.on) { display: none; }
.dfn-panel * { margin: 0; padding: 0; text-indent: 0; }
.dfn-panel > b { display: block; }
.dfn-panel a { color: black; }
.dfn-panel a:not(:hover) { text-decoration: none !important; border-bottom: none !important; }
.dfn-panel > b + b { margin-top: 0.25em; }
.dfn-panel > span { display: list-item; list-style: inside; }
.dfn-panel.activated {
display: inline-block;
position: fixed;
left: .5em;
bottom: .5em;
margin: 0 auto;
max-width: calc(100vw - 1.5em - .4em - .5em);
max-height: 30vh;
}
.dfn-paneled { cursor: pointer; }
</style>
<body class="h-entry">
<div class="head">
<p data-fill-with="logo"><a class="logo" href="http://www.w3.org/"> <img alt="W3C" height="48" src="https://www.w3.org/Icons/w3c_home" width="72"> </a> </p>
<h1 class="p-name no-ref" id="title">CSS Mobile Text Size Adjustment Module Level 1</h1>
<h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Editor’s Draft, <time class="dt-updated" datetime="2016-03-06">6 March 2016</time></span></h2>
<div data-fill-with="spec-metadata">
<dl>
<dt>This version:
<dd><a class="u-url" href="https://drafts.csswg.org/css-size-adjust/">https://drafts.csswg.org/css-size-adjust/</a>
<dt>Latest version:
<dd><a href="https://www.w3.org/TR/css-size-adjust/">https://www.w3.org/TR/css-size-adjust/</a>
<dt>Feedback:
<dd><span><a href="mailto:www-style@w3.org?subject=%5Bcss-size-adjust%5D%20YOUR%20TOPIC%20HERE">www-style@w3.org</a> with subject line “<kbd>[css-size-adjust] <i data-lt="">… message topic …</i></kbd>” (<a href="http://lists.w3.org/Archives/Public/www-style/" rel="discussion">archives</a>)</span>
<dt>Issue Tracking:
<dd><a href="#issues-index">Inline In Spec</a>
<dt class="editor">Editors:
<dd class="editor p-author h-card vcard"><a class="p-name fn u-url url" href="http://dbaron.org/">L. David Baron</a> (<a class="p-org org" href="https://www.mozilla.org/">Mozilla</a>)
<dd class="editor p-author h-card vcard"><a class="p-name fn u-url url" href="http://tantek.com/">Tantek Çelik</a> (<a class="p-org org" href="https://www.mozilla.org/">Mozilla</a>)
</dl>
</div>
<div data-fill-with="warning"></div>
<p class="copyright" data-fill-with="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a> © 2016 <a href="http://www.w3.org/"><abbr title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup> (<a href="http://www.csail.mit.edu/"><abbr title="Massachusetts Institute of Technology">MIT</abbr></a>, <a href="http://www.ercim.eu/"><abbr title="European Research Consortium for Informatics and Mathematics">ERCIM</abbr></a>, <a href="http://www.keio.ac.jp/">Keio</a>, <a href="http://ev.buaa.edu.cn/">Beihang</a>). W3C <a href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>, <a href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a> and <a href="http://www.w3.org/Consortium/Legal/copyright-documents">document use</a> rules apply. </p>
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
<div class="p-summary" data-fill-with="abstract">
<p>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>
<a href="http://www.w3.org/TR/CSS/">CSS</a> is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, in speech, etc.
</div>
<h2 class="no-num no-toc no-ref heading settled" id="status"><span class="content">Status of this document</span></h2>
<div data-fill-with="status">
<p> 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. </p>
<p> The (<a href="http://lists.w3.org/Archives/Public/www-style/">archived</a>) public mailing list <a href="mailto:www-style@w3.org?Subject=%5Bcss-size-adjust%5D%20PUT%20SUBJECT%20HERE">www-style@w3.org</a> (see <a href="http://www.w3.org/Mail/Request">instructions</a>)
is preferred for discussion of this specification.
When sending e-mail,
please put the text “css-size-adjust” in the subject,
preferably like this:
“[css-size-adjust] <em>…summary of comment…</em>” </p>
<p> This document was produced by the <a href="http://www.w3.org/Style/CSS/members">CSS Working Group</a> (part of the <a href="http://www.w3.org/Style/">Style Activity</a>). </p>
<p> This document was produced by a group operating under
the <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/">5 February 2004 W3C Patent Policy</a>.
W3C maintains a <a href="http://www.w3.org/2004/01/pp-impl/32061/status" rel="disclosure">public list of any patent disclosures</a> made in connection with the deliverables of the group;
that page also includes instructions for disclosing a patent.
An individual who has actual knowledge of a patent which the individual believes contains <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential">Essential Claim(s)</a> must disclose the information in accordance with <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure">section 6 of the W3C Patent Policy</a>. </p>
<p> This document is governed by the <a href="http://www.w3.org/2015/Process-20150901/" id="w3c_process_revision">1 September 2015 W3C Process Document</a>. </p>
<p></p>
<p>The following features are at risk:</p>
<ul>
<li>the reference to <a data-link-type="biblio" href="#biblio-css-text-4">[CSS-TEXT-4]</a>'s <a class="property" data-link-type="propdesc" href="https://drafts.csswg.org/css-text-4/#propdef-text-wrap">text-wrap</a> property
</ul>
</div>
<div data-fill-with="at-risk"></div>
<nav data-fill-with="table-of-contents" id="toc">
<h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
<ol class="toc" role="directory">
<li>
<a href="#intro"><span class="secno">1</span> <span class="content">Introduction</span></a>
<ol class="toc">
<li><a href="#placement"><span class="secno">1.1</span> <span class="content">Module interactions</span></a>
<li><a href="#values"><span class="secno">1.2</span> <span class="content">Values</span></a>
</ol>
<li>
<a href="#default-adjustment"><span class="secno">2</span> <span class="content">Default size adjustment</span></a>
<ol class="toc">
<li><a href="#default-adjustment-types"><span class="secno">2.1</span> <span class="content">Types of boxes adjusted</span></a>
<li><a href="#default-adjustment-conditions"><span class="secno">2.2</span> <span class="content">Conditions that suppress adjustment</span></a>
<li><a href="#default-adjustment-calculation"><span class="secno">2.3</span> <span class="content">Calculation of default adjustment</span></a>
</ol>
<li><a href="#adjustment-control"><span class="secno">3</span> <span class="content">Size adjustment control: the <span class="property">text-size-adjust</span> property</span></a>
<li><a href="#acknowledgments"><span class="secno">4</span> <span class="content">Acknowledgments</span></a>
<li>
<a href="#conformance"><span class="secno"></span> <span class="content"> Conformance</span></a>
<ol class="toc">
<li><a href="#document-conventions"><span class="secno"></span> <span class="content"> Document conventions</span></a>
<li><a href="#conform-classes"><span class="secno"></span> <span class="content"> Conformance classes</span></a>
<li>
<a href="#conform-responsible"><span class="secno"></span> <span class="content"> Requirements for Responsible Implementation of CSS</span></a>
<ol class="toc">
<li><a href="#conform-partial"><span class="secno"></span> <span class="content"> Partial Implementations</span></a>
<li><a href="#conform-future-proofing"><span class="secno"></span> <span class="content"> Implementations of Unstable and Proprietary Features</span></a>
<li><a href="#conform-testing"><span class="secno"></span> <span class="content"> Implementations of CR-level Features</span></a>
</ol>
</ol>
<li>
<a href="#index"><span class="secno"></span> <span class="content">Index</span></a>
<ol class="toc">
<li><a href="#index-defined-here"><span class="secno"></span> <span class="content">Terms defined by this specification</span></a>
<li><a href="#index-defined-elsewhere"><span class="secno"></span> <span class="content">Terms defined by reference</span></a>
</ol>
<li>
<a href="#references"><span class="secno"></span> <span class="content">References</span></a>
<ol class="toc">
<li><a href="#normative"><span class="secno"></span> <span class="content">Normative References</span></a>
<li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
</ol>
<li><a href="#property-index"><span class="secno"></span> <span class="content">Property Index</span></a>
<li><a href="#issues-index"><span class="secno"></span> <span class="content">Issues Index</span></a>
</ol>
</nav>
<main>
<h2 class="heading settled" data-level="1" id="intro"><span class="secno">1. </span><span class="content">Introduction</span><a class="self-link" href="#intro"></a></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 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 class="heading settled" data-level="1.1" id="placement"><span class="secno">1.1. </span><span class="content">Module interactions</span><a class="self-link" href="#placement"></a></h3>
<p>This module adds additional features that are not defined in <a data-link-type="biblio" href="#biblio-css21">[CSS21]</a>. These features may lead to a different size being computed
than would be computed when following <a data-link-type="biblio" href="#biblio-css21">[CSS21]</a> alone.</p>
<h3 class="heading settled" data-level="1.2" id="values"><span class="secno">1.2. </span><span class="content">Values</span><a class="self-link" href="#values"></a></h3>
<p>This specification follows the <a href="https://www.w3.org/TR/CSS21/about.html#property-defs">CSS property
definition conventions</a> from <a data-link-type="biblio" href="#biblio-css21">[CSS21]</a>. Value types not defined in
this specification are defined in CSS Level 2 Revision 1 <a data-link-type="biblio" href="#biblio-css21">[CSS21]</a>.
Other CSS modules may expand the definitions of these value types: for
example <a data-link-type="biblio" href="#biblio-css3color">[CSS3COLOR]</a>, when combined with this module, expands the
definition of the <color> 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="https://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.</p>
<h2 class="heading settled" data-level="2" id="default-adjustment"><span class="secno">2. </span><span class="content">Default size adjustment</span><a class="self-link" href="#default-adjustment"></a></h2>
<p>This section defines the default size adjustment rules. These rules
are then referenced by the definition of the <a class="property" data-link-type="propdesc" href="#propdef-text-size-adjust" id="ref-for-propdef-text-size-adjust-1">text-size-adjust</a> property
in the following section.</p>
<p class="issue" id="issue-9f6f751d"><a class="self-link" href="#issue-9f6f751d"></a> 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" id="issue-7ada1e3c"><a class="self-link" href="#issue-7ada1e3c"></a> 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 class="heading settled" data-level="2.1" id="default-adjustment-types"><span class="secno">2.1. </span><span class="content">Types of boxes adjusted</span><a class="self-link" href="#default-adjustment-types"></a></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 class="heading settled" data-level="2.2" id="default-adjustment-conditions"><span class="secno">2.2. </span><span class="content">Conditions that suppress adjustment</span><a class="self-link" href="#default-adjustment-conditions"></a></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 <a data-link-type="biblio" href="#biblio-css21">[CSS21]</a>) (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>when the objects to be adjusted are inside a block-level or <a class="css" data-link-type="propdesc" href="https://drafts.csswg.org/css2/visuren.html#propdef-display">display: inline-block</a> element with a <a class="property" data-link-type="propdesc" href="https://drafts.csswg.org/css2/visudet.html#propdef-height">height</a> other than <span class="css">auto</span> (see <a data-link-type="biblio" href="#biblio-css21">[CSS21]</a>),
<li>when the objects to be adjusted are inside a <a class="css" data-link-type="propdesc" href="https://drafts.csswg.org/css2/visuren.html#propdef-display">display: inline-block</a> element
with a <a class="property" data-link-type="propdesc" href="https://drafts.csswg.org/css2/visudet.html#propdef-width">width</a> other than <span class="css">auto</span> (see <a data-link-type="biblio" href="#biblio-css21">[CSS21]</a>),
<li>when the objects to be adjusted have <a class="property" data-link-type="propdesc" href="https://drafts.csswg.org/css-text-3/#propdef-white-space">white-space</a> of <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-text-3/#valdef-white-space-pre">pre</a> or <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-text-3/#valdef-white-space-nowrap">nowrap</a> (see <a data-link-type="biblio" href="#biblio-css21">[CSS21]</a>) or a <a class="property" data-link-type="propdesc" href="https://drafts.csswg.org/css-text-4/#propdef-text-wrap">text-wrap</a> of <span class="css">nowrap</span> (see <a data-link-type="biblio" href="#biblio-css-text-4">[CSS-TEXT-4]</a>).
</ul>
<h3 class="heading settled" data-level="2.3" id="default-adjustment-calculation"><span class="secno">2.3. </span><span class="content">Calculation of default adjustment</span><a class="self-link" href="#default-adjustment-calculation"></a></h3>
<p>The adjustment performed should be based on preferences (of the
renderer or the renderer’s user) indicating the desired <dfn data-dfn-type="dfn" data-noexport="" id="minimum-readable-text-size">minimum
readable text size<a class="self-link" href="#minimum-readable-text-size"></a></dfn>.
Given this preference, for each containing block of text to be adjusted,
there is a <dfn data-dfn-type="dfn" data-noexport="" id="minimum-block-text-size">minimum block text size<a class="self-link" href="#minimum-block-text-size"></a></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 <a class="property" data-link-type="propdesc" href="https://drafts.csswg.org/css-fonts-3/#propdef-font-size">font-size</a>. 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" id="issue-499161e9"><a class="self-link" href="#issue-499161e9"></a>Define
this with more detail/precision.</span></p>
<h2 class="heading settled" data-level="3" id="adjustment-control"><span class="secno">3. </span><span class="content">Size adjustment control: the <a class="property" data-link-type="propdesc" href="#propdef-text-size-adjust" id="ref-for-propdef-text-size-adjust-2">text-size-adjust</a> property</span><a class="self-link" href="#adjustment-control"></a></h2>
<table class="def propdef" data-link-for-hint="text-size-adjust">
<tbody>
<tr>
<th>Name:
<td><dfn class="dfn-paneled css" data-dfn-type="property" data-export="" data-lt="text-size-adjust" id="propdef-text-size-adjust">text-size-adjust<span class="dfn-panel" data-deco=""><b><a href="#propdef-text-size-adjust">#propdef-text-size-adjust</a></b><b>Referenced in:</b><span><a href="#ref-for-propdef-text-size-adjust-1">2. Default size adjustment</a></span><span><a href="#ref-for-propdef-text-size-adjust-2">3. Size adjustment control: the text-size-adjust property</a></span></span></dfn>
<tr class="value">
<th>Value:
<td class="prod">auto <a data-link-type="grammar" href="https://drafts.csswg.org/css-values-3/#comb-one">|</a> none <a data-link-type="grammar" href="https://drafts.csswg.org/css-values-3/#comb-one">|</a> <percentage>
<tr>
<th>Initial:
<td>auto
<tr>
<th>Applies to:
<td><a href="https://drafts.csswg.org/css-pseudo/#generated-content" title="Includes ::before and ::after pseudo-elements.">all elements</a>
<tr>
<th>Inherited:
<td>yes
<tr>
<th>Percentages:
<td>see below
<tr>
<th>Media:
<td>visual
<tr>
<th>Computed value:
<td>as specified
<tr>
<th>Animatable:
<td>as percentage
<tr>
<th>Canonical order:
<td>N/A
</table>
<dl>
<dt><dfn class="css" data-dfn-for="text-size-adjust" data-dfn-type="value" data-export="" id="valdef-text-size-adjust-auto">auto<a class="self-link" href="#valdef-text-size-adjust-auto"></a></dfn>
<dd>Renderers must use the <a href="#default-adjustment">default size adjustment</a> when displaying on a small device.
<dt><dfn class="css" data-dfn-for="text-size-adjust" data-dfn-type="value" data-export="" id="valdef-text-size-adjust-none">none<a class="self-link" href="#valdef-text-size-adjust-none"></a></dfn>
<dd>Renderers must not do size adjustment when displaying on a small device.
<dt><dfn class="css" data-dfn-for="text-size-adjust" data-dfn-type="value" data-export="" id="valdef-text-size-adjust-percentage"><percentage><a class="self-link" href="#valdef-text-size-adjust-percentage"></a></dfn>
<dd>
When displaying on a small device, renderers must not do size adjustment but instead the computed value of <a class="property" data-link-type="propdesc" href="https://drafts.csswg.org/css-fonts-3/#propdef-font-size">font-size</a> must be multiplied by this percentage.
<p class="note" role="note">Note: This means that 'text-size-adjust: 100%;' is equivalent to 'text-size-adjust: none;'.</p>
</dl>
<p class="issue" id="issue-0028bf4a"><a class="self-link" href="#issue-0028bf4a"></a> "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.</p>
<h2 class="heading settled" data-level="4" id="acknowledgments"><span class="secno">4. </span><span class="content">Acknowledgments</span><a class="self-link" href="#acknowledgments"></a></h2>
<p>The editors would like to thank:
.</p>
</main>
<h2 class="no-ref no-num heading settled" id="conformance"><span class="content"> Conformance</span><a class="self-link" href="#conformance"></a></h2>
<h3 class="heading settled" id="document-conventions"><span class="content"> Document conventions</span><a class="self-link" href="#document-conventions"></a></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>
<p>All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. <a data-link-type="biblio" href="#biblio-rfc2119">[RFC2119]</a> </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: </p>
<div class="example" id="example-52448c84">
<a class="self-link" href="#example-52448c84"></a>
<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>
<p class="note" role="note">Note, this is an informative note. </p>
<p>Advisements are normative sections styled to evoke special attention and are
set apart from other normative text with <code><strong class="advisement"></code>, like
this: <strong class="advisement"> UAs MUST provide an accessible alternative. </strong> </p>
<h3 class="heading settled" id="conform-classes"><span class="content"> Conformance classes</span><a class="self-link" href="#conform-classes"></a></h3>
<p>Conformance to this specification
is defined for three conformance classes: </p>
<dl>
<dt>style sheet
<dd>A <a href="http://www.w3.org/TR/CSS2/conform.html#style-sheet">CSS
style sheet</a>.
<dt>renderer
<dd>A <a href="http://www.w3.org/TR/CSS2/conform.html#user-agent">UA</a> that interprets the semantics of a style sheet and renders
documents that use them.
<dt>authoring tool
<dd>A <a href="http://www.w3.org/TR/CSS2/conform.html#user-agent">UA</a> that writes a style sheet.
</dl>
<p>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. </p>
<p>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.) </p>
<p>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. </p>
<h3 class="heading settled" id="conform-responsible"><span class="content"> Requirements for Responsible Implementation of CSS</span><a class="self-link" href="#conform-responsible"></a></h3>
<p>The following sections define several conformance requirements
for implementing CSS responsibly,
in a way that promotes interoperability in the present and future. </p>
<h4 class="heading settled" id="conform-partial"><span class="content"> Partial Implementations</span><a class="self-link" href="#conform-partial"></a></h4>
<p>So that authors can exploit the forward-compatible parsing rules to assign fallback values, <strong>CSS renderers <em>must</em> treat as invalid
(and <a href="http://www.w3.org/TR/CSS2/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</strong>.
In particular, user agents <em>must not</em> selectively ignore
unsupported property 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>
<h4 class="heading settled" id="conform-future-proofing"><span class="content"> Implementations of Unstable and Proprietary Features</span><a class="self-link" href="#conform-future-proofing"></a></h4>
<p>To avoid clashes with future stable CSS features,
the CSSWG recommends <a href="http://www.w3.org/TR/CSS/#future-proofing">following best practices</a> for the implementation of <a href="http://www.w3.org/TR/CSS/#unstable">unstable</a> features and <a href="http://www.w3.org/TR/CSS/#proprietary-extension">proprietary extensions</a> to CSS. </p>
<h4 class="heading settled" id="conform-testing"><span class="content"> Implementations of CR-level Features</span><a class="self-link" href="#conform-testing"></a></h4>
<p>Once a specification reaches the Candidate Recommendation stage,
implementers should release an <a data-link-type="dfn" href="http://www.w3.org/TR/CSS/#vendor-prefix">unprefixed</a> implementation
of any CR-level feature they can demonstrate
to be correctly implemented according to spec,
and should avoid exposing a prefixed variant of that feature. </p>
<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>
<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.
<script src="https://www.w3.org/scripts/TR/2016/fixup.js"></script>
</p>
<h2 class="no-num no-ref heading settled" id="index"><span class="content">Index</span><a class="self-link" href="#index"></a></h2>
<h3 class="no-num no-ref heading settled" id="index-defined-here"><span class="content">Terms defined by this specification</span><a class="self-link" href="#index-defined-here"></a></h3>
<ul class="index">
<li><a href="#valdef-text-size-adjust-auto">auto</a><span>, in §3</span>
<li><a href="#minimum-block-text-size">minimum block text size</a><span>, in §2.3</span>
<li><a href="#minimum-readable-text-size">minimum
readable text size</a><span>, in §2.3</span>
<li><a href="#valdef-text-size-adjust-none">none</a><span>, in §3</span>
<li><a href="#valdef-text-size-adjust-percentage"><percentage></a><span>, in §3</span>
<li><a href="#propdef-text-size-adjust">text-size-adjust</a><span>, in §3</span>
</ul>
<h3 class="no-num no-ref heading settled" id="index-defined-elsewhere"><span class="content">Terms defined by reference</span><a class="self-link" href="#index-defined-elsewhere"></a></h3>
<ul class="index">
<li>
<a data-link-type="biblio" href="#biblio-css-fonts-3">[css-fonts-3]</a> defines the following terms:
<ul>
<li><a href="https://drafts.csswg.org/css-fonts-3/#propdef-font-size">font-size</a>
</ul>
<li>
<a data-link-type="biblio" href="#biblio-css-text-3">[css-text-3]</a> defines the following terms:
<ul>
<li><a href="https://drafts.csswg.org/css-text-3/#valdef-white-space-nowrap">nowrap</a>
<li><a href="https://drafts.csswg.org/css-text-3/#valdef-white-space-pre">pre</a>
<li><a href="https://drafts.csswg.org/css-text-3/#propdef-white-space">white-space</a>
</ul>
<li>
<a data-link-type="biblio" href="#biblio-css-text-4">[css-text-4]</a> defines the following terms:
<ul>
<li><a href="https://drafts.csswg.org/css-text-4/#propdef-text-wrap">text-wrap</a>
</ul>
<li>
<a data-link-type="biblio" href="#biblio-css-values">[css-values]</a> defines the following terms:
<ul>
<li><a href="https://drafts.csswg.org/css-values-3/#comb-one">|</a>
</ul>
<li>
<a data-link-type="biblio" href="#biblio-css21">[CSS21]</a> defines the following terms:
<ul>
<li><a href="https://drafts.csswg.org/css2/visuren.html#propdef-display">display</a>
<li><a href="https://drafts.csswg.org/css2/visudet.html#propdef-height">height</a>
<li><a href="https://drafts.csswg.org/css2/visudet.html#propdef-width">width</a>
</ul>
</ul>
<h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
<h3 class="no-num no-ref heading settled" id="normative"><span class="content">Normative References</span><a class="self-link" href="#normative"></a></h3>
<dl>
<dt id="biblio-css-fonts-3">[CSS-FONTS-3]
<dd>John Daggett. <a href="http://dev.w3.org/csswg/css-fonts/">CSS Fonts Module Level 3</a>. 3 October 2013. CR. URL: <a href="http://dev.w3.org/csswg/css-fonts/">http://dev.w3.org/csswg/css-fonts/</a>
<dt id="biblio-css-text-3">[CSS-TEXT-3]
<dd>Elika Etemad; Koji Ishii. <a href="http://dev.w3.org/csswg/css-text-3/">CSS Text Module Level 3</a>. 10 October 2013. LCWD. URL: <a href="http://dev.w3.org/csswg/css-text-3/">http://dev.w3.org/csswg/css-text-3/</a>
<dt id="biblio-css-text-4">[CSS-TEXT-4]
<dd>Elika Etemad; Koji Ishii; Alan Stearns. <a href="http://www.w3.org/TR/css-text-4/">CSS Text Module Level 4</a>. 22 September 2015. WD. URL: <a href="http://www.w3.org/TR/css-text-4/">http://www.w3.org/TR/css-text-4/</a>
<dt id="biblio-css-values">[CSS-VALUES]
<dd>Tab Atkins Jr.; Elika Etemad. <a href="http://dev.w3.org/csswg/css-values/">CSS Values and Units Module Level 3</a>. 11 June 2015. CR. URL: <a href="http://dev.w3.org/csswg/css-values/">http://dev.w3.org/csswg/css-values/</a>
<dt id="biblio-css21">[CSS21]
<dd>Bert Bos; et al. <a href="http://www.w3.org/TR/CSS2">Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification</a>. 7 June 2011. REC. URL: <a href="http://www.w3.org/TR/CSS2">http://www.w3.org/TR/CSS2</a>
<dt id="biblio-rfc2119">[RFC2119]
<dd>S. Bradner. <a href="https://tools.ietf.org/html/rfc2119">Key words for use in RFCs to Indicate Requirement Levels</a>. March 1997. Best Current Practice. URL: <a href="https://tools.ietf.org/html/rfc2119">https://tools.ietf.org/html/rfc2119</a>
</dl>
<h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
<dl>
<dt id="biblio-css3color">[CSS3COLOR]
<dd>Tantek Çelik; Chris Lilley; David Baron. <a href="http://www.w3.org/TR/css3-color">CSS Color Module Level 3</a>. 7 June 2011. REC. URL: <a href="http://www.w3.org/TR/css3-color">http://www.w3.org/TR/css3-color</a>
</dl>
<h2 class="no-num no-ref heading settled" id="property-index"><span class="content">Property Index</span><a class="self-link" href="#property-index"></a></h2>
<div class="big-element-wrapper">
<table class="index">
<thead>
<tr>
<th scope="col">Name
<th scope="col">Value
<th scope="col">Initial
<th scope="col">Applies to
<th scope="col">Inh.
<th scope="col">%ages
<th scope="col">Media
<th scope="col">Animatable
<th scope="col">Canonical order
<th scope="col">Computed value
<tbody>
<tr>
<th scope="row"><a class="css" data-link-type="property" href="#propdef-text-size-adjust">text-size-adjust</a>
<td>auto | none | <percentage>
<td>auto
<td>all elements
<td>yes
<td>see below
<td>visual
<td>as percentage
<td>N/A
<td>as specified
</table>
</div>
<h2 class="no-num no-ref heading settled" id="issues-index"><span class="content">Issues Index</span><a class="self-link" href="#issues-index"></a></h2>
<div style="counter-reset:issue">
<div class="issue"> All of the subsections of this section need significant
refinement: additional detail, verification that the detail already
present is correct, etc.<a href="#issue-9f6f751d"> ↵ </a></div>
<div 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.<a href="#issue-7ada1e3c"> ↵ </a></div>
<div class="issue">Define
this with more detail/precision.<a href="#issue-499161e9"> ↵ </a></div>
<div class="issue"> "
5C49
;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.<a href="#issue-0028bf4a"> ↵ </a></div>
</div>
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