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<!doctype html><html lang="en">
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<title>CSS Scroll Snap Points Module Level 1</title>
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<body class="h-entry">
<div class="head">
<p data-fill-with="logo"><a href="http://www.w3.org/"> <img alt="W3C" height="48" src="https://www.w3.org/StyleSheets/TR/2016/logos/W3C" width="72"> </a> </p>
<h1 class="p-name no-ref" id="title">CSS Scroll Snap Points Module Level 1</h1>
<h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">W3C Working Draft, <time class="dt-updated" datetime="2016-03-29">29 March 2016</time></span></h2>
<div data-fill-with="spec-metadata">
<dl>
<dt>This version:
<dd><a class="u-url" href="http://www.w3.org/TR/2016/WD-css-snappoints-1-20160329/">http://www.w3.org/TR/2016/WD-css-snappoints-1-20160329/</a>
<dt>Latest version:
<dd><a href="http://www.w3.org/TR/css-snappoints-1/">http://www.w3.org/TR/css-snappoints-1/</a>
<dt>Editor's Draft:
<dd><a href="https://drafts.csswg.org/css-snappoints/">https://drafts.csswg.org/css-snappoints/</a>
<dt>Previous Versions:
<dd><a href="http://www.w3.org/TR/2015/WD-css-snappoints-1-20150326/" rel="previous">http://www.w3.org/TR/2015/WD-css-snappoints-1-20150326/</a>
<dt>Feedback:
<dd><span><a href="mailto:www-style@w3.org?subject=%5Bcss-snappoints%5D%20YOUR%20TOPIC%20HERE">www-style@w3.org</a> with subject line “<kbd>[css-snappoints] <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"><span class="p-name fn">Matt Rakow</span> (<span class="p-org org">Microsoft</span>)
<dd class="editor p-author h-card vcard"><span class="p-name fn">Jacob Rossi</span> (<span class="p-org org">Microsoft</span>)
</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 to control panning and scrolling behavior with "snap positions".</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><em>This section describes the status of this document at the time of
its publication. Other documents may supersede this document. A list of
current W3C publications and the latest revision of this technical report
can be found in the <a href="http://www.w3.org/TR/">W3C technical reports
index at http://www.w3.org/TR/.</a></em> </p>
<p>Publication as a Working Draft does not imply endorsement by the W3C
Membership. This is a draft document and may be updated, replaced or
obsoleted by other documents at any time. It is inappropriate to cite this
document as other than 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">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-snappoints” in the subject, preferably like this:
“[css-snappoints] <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>
</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="#examples"><span class="secno">2</span> <span class="content">Motivating Examples</span></a>
<li><a href="#overview"><span class="secno">3</span> <span class="content">Overview</span></a>
<li><a href="#definitions"><span class="secno">4</span> <span class="content">Definitions</span></a>
<li>
<a href="#properties-on-the-scroll-container"><span class="secno">5</span> <span class="content">Properties on the scroll container</span></a>
<ol class="toc">
<li><a href="#scroll-snap-type"><span class="secno">5.1</span> <span class="content">Scroll Snap Types: the <span class="property">scroll-snap-type</span> property</span></a>
<li><a href="#scroll-snap-padding"><span class="secno">5.2</span> <span class="content">Scroll Snap Padding: the <span class="property">scroll-snap-padding</span> property</span></a>
</ol>
<li>
<a href="#properties-on-the-elements"><span class="secno">6</span> <span class="content">Properties on the elements</span></a>
<ol class="toc">
<li><a href="#scroll-snap-margin0"><span class="secno">6.1</span> <span class="content">Scroll Snap Margin: the <span class="property">scroll-snap-margin</span> property</span></a>
<li><a href="#scroll-snap-align"><span class="secno">6.2</span> <span class="content">Scroll Snap Alignment: the <span class="property">scroll-snap-align</span> property</span></a>
</ol>
<li><a href="#acknowledgements"><span class="secno">7</span> <span class="content">Acknowledgements</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>
</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>
<p class="issue" id="issue-752e30a5"><a class="self-link" href="#issue-752e30a5"></a> <b>This spec is currently undergoing major changes in line with the resolutions in <a href="https://lists.w3.org/Archives/Public/www-style/2015Dec/0048.html">https://lists.w3.org/Archives/Public/www-style/2015Dec/0048.html</a>. During this process it is not safe to assume that the current form will be representative of what it will look like in the future. After the work in process is completed, it will closely resemble <a href="https://drafts.csswg.org/css-scroll-snap/">this proposal</a>.</b></p>
<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><em>This section is not normative.</em></p>
<p>Popular UX paradigms for scrollable content frequently employ paging through content,
or sectioning into logical divisions.
This is especially true for touch interactions
where it is quicker and easier for users to quickly pan through a flatly-arranged breadth of content
rather than delving into a heirarchical structure through tap navigation.
For example, it is easier for a user to view many photos in a photo album
by panning through a photo slideshow view
rather than tapping on individual photos in an album.</p>
<p>However, given the imprecise nature of scrolling inputs
like touch panning and mousewheel scrolling,
it is difficult for web developers to guarantee a well-controlled scrolling experience,
in particular creating the effect of paging through content.
For instance, it is easy for a user to land at an awkward scroll offset
which leaves a page partially on-screen when panning.</p>
<p>To this end, we introduce scroll snap positions
which enforce the scroll offsets that a <a data-link-type="dfn" href="#scroll-container" id="ref-for-scroll-container-1">scroll container’s</a> visual viewport may end at
after a scrolling operation has completed.</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 extends the scrolling user interface features defined in <a data-link-type="biblio" href="#biblio-css21">[CSS21]</a> section 11.1.</p>
<p>None of the properties in this module apply to the <a class="css" data-link-type="maybe" href="https://www.w3.org/TR/css-pseudo-4/#selectordef-first-line">::first-line</a> and <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-pseudo-4/#selectordef-first-letter">::first-letter</a> pseudo-elements.</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-css3val">[CSS3VAL]</a>, when combined with this module, expands the
definition of the <a class="production css" data-link-type="type" href="https://www.w3.org/TR/css3-values/#length-value"><length></a> value type as used in this specification.</p>
<h2 class="heading settled" data-level="2" id="examples"><span class="secno">2. </span><span class="content">Motivating Examples</span><a class="self-link" href="#examples"></a></h2>
<div class="example" id="example-b17d46c1">
<a class="self-link" href="#example-b17d46c1"></a> In this example, a series of images arranged in a <a data-link-type="dfn" href="#scroll-container" id="ref-for-scroll-container-2">scroll container</a> are used to build a photo gallery. In this example the <a data-link-type="dfn" href="#scroll-container" id="ref-for-scroll-container-3">scroll container</a> is larger than the photos contained within (such that multiple images may be seen simultaneously), and the image
sizes vary. Using mandatory element-based snap
positions, scrolling will always complete with an image centered in the <a data-link-type="dfn" href="#scroll-container" id="ref-for-scroll-container-4">scroll container’s</a> visual viewport.
<pre class="lang-css highlight"><span class="nt">img </span><span class="p">{</span>
<span class="c">/* Specifies that the center of each photo </span>
<span class="c"> should align with the center of the scroll </span>
<span class="c"> container in the X axis when snapping */</span>
<span class="k">scroll-snap-align</span><span class="p">:</span> center none<span class="p">;</span>
<span class="p">}</span>
<span class="nt">.photoGallery </span><span class="p">{</span>
<span class="k">width</span><span class="p">:</span> <span class="m">500</span><span class="l">px</span><span class="p">;</span>
<span class="k">overflow-x</span><span class="p">:</span> auto<span class="p">;</span>
<span class="k">overflow-y</span><span class="p">:</span> hidden<span class="p">;</span>
<span class="k">white-space</span><span class="p">:</span> nowrap<span class="p">;</span>
<span class="c">/* Requires that the scroll offset always be </span>
<span class="c"> at a valid snap position when the scrolling </span>
<span class="c"> operation completes. */</span>
<span class="k">scroll-snap-type</span><span class="p">:</span> mandatory<span class="p">;</span>
<span class="p">}</span></pre>
<pre class="lang-html highlight"><span class="nt"><div</span> <span class="na">class=</span><span class="s">"photoGallery"</span><span class="nt">></span>
<span class="nt"><img</span> <span class="na">src=</span><span class="s">"img1.jpg"</span><span class="nt">></span>
<span class="nt"><img</span> <span class="na">src=</span><span class="s">"img2.jpg"</span><span class="nt">></span>
<span class="nt"><img</span> <span class="na">src=</span><span class="s">"img3.jpg"</span><span class="nt">></span>
<span class="nt"><img</span> <span class="na">src=</span><span class="s">"img4.jpg"</span><span class="nt">></span>
<span class="nt"><img</span> <span class="na">src=</span><span class="s">"img5.jpg"</span><span class="nt">></span>
<span class="nt"></div></span></pre>
<figure>
<img alt="" src="element_snap_positions.png">
<figcaption> The layout of the scroll container’s contents in the example.
The snap alignment container is represented by the red rectangle, and the snap margin is represented by the yellow rectangle. Since the scroll-snap-align is "center" in the X axis, a snap position is established at each scroll offset which aligns the X-center of the snap alignment container (represented by a red dotted line) with the X-center of a snap margin (represented by a yellow dotted line). </figcaptio
8097
n>
</figure>
</div>
<div class="example" id="example-5b068871">
<a class="self-link" href="#example-5b068871"></a> This example builds a paginated document that aligns each page near to (but not exactly on) the edge of the <a data-link-type="dfn" href="#scroll-container" id="ref-for-scroll-container-5">scroll container</a>.
This allows the previous page to "peek" in from above in order to make the user aware that they are not yet at the top of the document.
Using proximity snap positions instead of mandatory snap positions allows the user to stop halfway through a page (rather than forcing them
to snap one page at a time). However, if a scrolling operation would finish near a snap position, then the scroll will be adjusted to
align the page as specified.
<pre class="lang-css highlight"><span class="nt">.page </span><span class="p">{</span>
<span class="c">/* Defines the top of each page as the</span>
<span class="c"> edge that should be used for snapping */</span>
<span class="k">scroll-snap-align</span><span class="p">:</span> none start<span class="p">;</span>
<span class="p">}</span>
<span class="nt">.docScroller </span><span class="p">{</span>
<span class="k">width</span><span class="p">:</span> <span class="m">500</span><span class="l">px</span><span class="p">;</span>
<span class="k">overflow-x</span><span class="p">:</span> hidden<span class="p">;</span>
<span class="k">overflow-y</span><span class="p">:</span> auto<span class="p">;</span>
<span class="c">/* Specifies that each element’s snap margin should</span>
<span class="c"> align with a 100px offset from the top edge. */</span>
<span class="k">scroll-snap-padding</span><span class="p">:</span> <span class="m">100</span><span class="l">px</span> <span class="m">0</span> <span class="m">0</span><span class="p">;</span>
<span class="c">/* Encourages scrolling to end at a snap position when the</span>
<span class="c"> operation completes, if it is near a valid snap position */</span>
<span class="k">scroll-snap-type</span><span class="p">:</span> proximity<span class="p">;</span>
<span class="p">}</span></pre>
<pre class="lang-html highlight"><span class="nt"><div</span> <span class="na">class=</span><span class="s">"docScroller"</span><span class="nt">></span>
<span class="nt"><div</span> <span class="na">class=</span><span class="s">"page"</span><span class="nt">></span>Page 1<span class="nt"></div></span>
<span class="nt"><div</span> <span class="na">class=</span><span class="s">"page"</span><span class="nt">></span>Page 2<span class="nt"></div></span>
<span class="nt"><div</span> <span class="na">class=</span><span class="s">"page"</span><span class="nt">></span>Page 3<span class="nt"></div></span>
<span class="nt"><div</span> <span class="na">class=</span><span class="s">"page"</span><span class="nt">></span>Page 4<span class="nt"></div></span>
<span class="nt"></div></span></pre>
<figure>
<img alt="" src="element_snap_positions_offset.png">
<figcaption> The layout of the scroll container’s contents in the example.
The snap alignment container is represented by the red rectangle (inset from the top by 100px due to the scroll-snap-padding), and the snap margin is represented by the yellow rectangle. Since the scroll-snap-align is "start" in the Y axis, a snap position is established at each scroll offset which aligns the Y-start of the snap alignment container (represented by a red dotted line) with the Y-start of a snap margin (represented by a yellow dotted line). </figcaption>
</figure>
</div>
<h2 class="heading settled" data-level="3" id="overview"><span class="secno">3. </span><span class="content">Overview</span><a class="self-link" href="#overview"></a></h2>
<p>This module introduces control over <a data-link-type="dfn" href="#scroll-snap-position" id="ref-for-scroll-snap-position-1">scroll snap positions</a>,
which are scroll positions that produce particular alignments
of content within a scrollable viewport.
Using the <a class="property" data-link-type="propdesc" href="#propdef-scroll-snap-type" id="ref-for-propdef-scroll-snap-type-1">scroll-snap-type</a> property on the relevant <a data-link-type="dfn" href="#scroll-container" id="ref-for-scroll-container-6">scroll container</a>,
the author can request a particular bias
for the viewport to land on a valid <a data-link-type="dfn" href="#scroll-snap-position" id="ref-for-scroll-snap-position-2">snap position</a> after scrolling operations.</p>
<p>Valid <a data-link-type="dfn" href="#scroll-snap-position" id="ref-for-scroll-snap-position-3">snap positions</a> can be specified
as a particular alignment (<a class="property" data-link-type="propdesc" href="#propdef-scroll-snap-align" id="ref-for-propdef-scroll-snap-align-1">scroll-snap-align</a>)
of an element’s <a data-link-type="dfn" href="#scroll-snap-margin" id="ref-for-scroll-snap-margin-1">scroll snap margin</a> (<a class="property" data-link-type="propdesc" href="#propdef-scroll-snap-margin" id="ref-for-propdef-scroll-snap-margin-1">scroll-snap-margin</a>, defaulting to its margin box)
within the <a data-link-type="dfn" href="#scroll-container" id="ref-for-scroll-container-7">scroll container</a>’s <a data-link-type="dfn" href="#snap-alignment-container" id="ref-for-snap-alignment-container-1">snap alignment container</a> (the rectangle obtained by reducing its visual viewport by its <a class="property" data-link-type="propdesc" href="#propdef-scroll-snap-padding" id="ref-for-propdef-scroll-snap-padding-1">scroll-snap-padding</a>).
This is conceptually equivalent to specifying the alignment of
an <a data-link-type="dfn" href="https://www.w3.org/TR/css3-align/#alignment-subject">alignment subject</a> within an <a data-link-type="dfn" href="https://www.w3.org/TR/css3-align/#alignment-container">alignment container</a>.
A scroll position that satisfies the specified alignment
is a valid <a data-link-type="dfn" href="#scroll-snap-position" id="ref-for-scroll-snap-position-4">snap position</a>.</p>
<p><a data-link-type="dfn" href="#scroll-snap-position" id="ref-for-scroll-snap-position-5">Snap positions</a> must only affect the nearest ancestor
(on the element’s <a data-link-type="dfn" href="https://drafts.csswg.org/css-display-3/#containing-block-chain">containing block chain</a>) <a data-link-type="dfn" href="#scroll-container" id="ref-for-scroll-container-8">scroll container</a>.</p>
<h2 class="heading settled" data-level="4" id="definitions"><span class="secno">4. </span><span class="content">Definitions</span><a class="self-link" href="#definitions"></a></h2>
<dl>
<dt><dfn class="dfn-paneled" data-dfn-type="dfn" data-export="" data-lt="scroll container" id="scroll-container">scroll container<span class="dfn-panel" data-deco=""><b><a href="#scroll-container">#scroll-container</a></b><b>Referenced in:</b><span><a href="#ref-for-scroll-container-1">1. Introduction</a></span><span><a href="#ref-for-scroll-container-2">2. Motivating Examples</a> <a href="#ref-for-scroll-container-3">(2)</a> <a href="#ref-for-scroll-container-4">(3)</a> <a href="#ref-for-scroll-container-5">(4)</a></span><span><a href="#ref-for-scroll-container-6">3. Overview</a> <a href="#ref-for-scroll-container-7">(2)</a> <a href="#ref-for-scroll-container-8">(3)</a></span><span><a href="#ref-for-scroll-container-9">5.1. Scroll Snap Types: the scroll-snap-type property</a> <a href="#ref-for-scroll-container-10">(2)</a> <a href="#ref-for-scroll-container-11">(3)</a> <a href="#ref-for-scroll-container-12">(4)</a></span><span><a href="#ref-for-scroll-container-13">5.2. Scroll Snap Padding: the scroll-snap-padding property</a></span><span><a href="#ref-for-scroll-container-14">6.1. Scroll Snap Margin: the scroll-snap-margin property</a> <a href="#ref-for-scroll-container-15">(2)</a> <a href="#ref-for-scroll-container-16">(3)</a></span></span></dfn>
<dd> An element which provides a scrolling user interface as described in <a data-link-type="biblio" href="#biblio-css21">[CSS21]</a>, particularly in the section on overflow.
<dt><dfn class="dfn-paneled" data-dfn-type="dfn" data-export="" data-lt="snap alignment container" id="snap-alignment-container">snap alignment container<span class="dfn-panel" data-deco=""><b><a href="#snap-alignment-container">#snap-alignment-container</a></b><b>Referenced in:</b><span><a href="#ref-for-snap-alignment-container-1">3. Overview</a></span><span><a href="#ref-for-snap-alignment-container-2">5.2. Scroll Snap Padding: the scroll-snap-padding property</a></span></span></dfn>
<dd>
A scroll container’s snap alignment container is the rectangle obtained by reducing its visual viewport by its <a class="property" data-link-type="propdesc" href="#propdef-scroll-snap-padding" id="ref-for-propdef-scroll-snap-padding-2">scroll-snap-padding</a>.
<p class="issue" id="issue-1b6eda6b"><a class="self-link" href="#issue-1b6eda6b"></a> Better name for this concept?</p>
<dt><dfn class="dfn-paneled" data-dfn-type="dfn" data-export="" data-local-lt="snap margin" data-lt="scroll snap margin" id="scroll-snap-margin">snap margin<span class="dfn-panel" data-deco=""><b><a href="#scroll-snap-margin">#scroll-snap-margin</a></b><b>Referenced in:</b><span><a href="#ref-for-scroll-snap-margin-1">3. Overview</a></span><span><a href="#ref-for-scroll-snap-margin-2">6.1. Scroll Snap Margin: the scroll-snap-margin property</a> <a href="#ref-for-scroll-snap-margin-3">(2)</a></span></span></dfn>
<dd> An element’s snap margin is the rectangle obtained by expanding its border box by its <a class="property" data-link-type="propdesc" href="#propdef-scroll-snap-margin" id="ref-for-propdef-scroll-snap-margin-2">scroll-snap-margin</a>.
<dt><dfn class="dfn-paneled" data-dfn-type="dfn" data-export="" data-local-lt="snap position" data-lt="scroll snap position" id="scroll-snap-position">snap position<span class="dfn-panel" data-deco=""><b><a href="#scroll-snap-position">#scroll-snap-position</a></b><b>Referenced in:</b><span><a href="#ref-for-scroll-snap-position-1">3. Overview</a> <a href="#ref-for-scroll-snap-position-2">(2)</a> <a href="#ref-for-scroll-snap-position-3">(3)</a> <a href="#ref-for-scroll-snap-position-4">(4)</a> <a href="#ref-for-scroll-snap-position-5">(5)</a></span><span><a href="#ref-for-scroll-snap-position-6">5.1. Scroll Snap Types: the scroll-snap-type property</a></span><span><a href="#ref-for-scroll-snap-position-7">5.2. Scroll Snap Padding: the scroll-snap-padding property</a> <a href="#ref-for-scroll-snap-position-8">(2)</a></span><span><a href="#ref-for-scroll-snap-position-9">6.1. Scroll Snap Margin: the scroll-snap-margin property</a></span><span><a href="#ref-for-scroll-snap-position-10">6.2. Scroll Snap Alignment: the scroll-snap-align property</a> <a href="#ref-for-scroll-snap-position-11">(2)</a> <a href="#ref-for-scroll-snap-position-12">(3)</a> <a href="#ref-for-scroll-snap-position-13">(4)</a></span></span></dfn>
<dd> For a scroll container, a particular value for its scroll offset is a snap position if when scrolled to that offset the visual viewport of the scroll container would align with a descendent element in the manner specified by the scroll snap properties.
</dl>
<h2 class="heading settled" data-level="5" id="properties-on-the-scroll-container"><span class="secno">5. </span><span class="content">Properties on the scroll container</span><a class="self-link" href="#properties-on-the-scroll-container"></a></h2>
<h3 class="heading settled" data-level="5.1" id="scroll-snap-type"><span class="secno">5.1. </span><span class="content">Scroll Snap Types: the <a class="property" data-link-type="propdesc" href="#propdef-scroll-snap-type" id="ref-for-propdef-scroll-snap-type-2">scroll-snap-type</a> property</span><a class="self-link" href="#scroll-snap-type"></a></h3>
<p>The <a class="property" data-link-type="propdesc" href="#propdef-scroll-snap-type" id="ref-for-propdef-scroll-snap-type-3">scroll-snap-type</a> property defines how strictly a <a data-link-type="dfn" href="#scroll-container" id="ref-for-scroll-container-9">scroll container’s</a> visual viewport should rest on <a data-link-type="dfn" href="#scroll-snap-position" id="ref-for-scroll-snap-position-6">snap positions</a>. It intentionally does not specify nor mandate any precise animations or physics used to enforce those snap positions; this is left up to the user agent.</p>
<p class="issue" id="issue-7e0cf181"><a class="self-link" href="#issue-7e0cf181"></a> <a href="https://lists.w3.org/Archives/Public/www-style/2015Nov/0328.html">Open issue</a> on whether to enhance the scroll-snap-type property for specifying the axis or adding a second property. We have <a href="https://lists.w3.org/Archives/Public/www-style/2015Dec/0048.html">resolved</a> that this functionality be added once the issue is resolved.</p>
<p class="issue" id="issue-780b32d8"><a class="self-link" href="#issue-780b32d8"></a> Currently considering renaming the values</p>
<table class="def propdef" data-link-for-hint="scroll-snap-type">
<tbody>
<tr>
<th>Name:
<td><dfn class="dfn-paneled css" data-dfn-type="property" data-export="" data-lt="scroll-snap-type" id="propdef-scroll-snap-type">scroll-snap-type<span class="dfn-panel" data-deco=""><b><a href="#propdef-scroll-snap-type">#propdef-scroll-snap-type</a></b><b>Referenced in:</b><span><a href="#ref-for-propdef-scroll-snap-type-1">3. Overview</a></span><span><a href="#ref-for-propdef-scroll-snap-type-2">5.1. Scroll Snap Types: the scroll-snap-type property</a> <a href="#ref-for-propdef-scroll-snap-type-3">(2)</a></span></span></dfn>
<tr class="value">
<th>Value:
<td class="prod">none <a data-link-type="grammar" href="https://www.w3.org/TR/css3-values/#comb-one">|</a> mandatory <a data-link-type="grammar" href="https://www.w3.org/TR/css3-values/#comb-one">|</a> proximity
<tr>
<th>Initial:
<td>none
<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>no
<tr>
<th>Percentages:
<td>n/a
<tr>
<th>Media:
<td>interactive
<tr>
<th>Computed value:
<td>as specified
<tr>
<th>Animatable:
<td>no
</table>
<dl>
<dt><dfn class="css" data-dfn-for="scroll-snap-type" data-dfn-type="value" data-export="" id="valdef-scroll-snap-type-none">none<a class="self-link" href="#valdef-scroll-snap-type-none"></a></dfn>
<dd> The visual viewport of this <a data-link-type="dfn" href="#scroll-container" id="ref-for-scroll-container-10">scroll container</a> must ignore snap positions.
<dt><dfn class="css" data-dfn-for="scroll-snap-type" data-dfn-type="value" data-export="" id="valdef-scroll-snap-type-mandatory">mandatory<a class="self-link" href="#valdef-scroll-snap-type-mandatory"></a></dfn>
<dd>
The visual viewport of this <a data-link-type="dfn" href="#scroll-container" id="ref-for-scroll-container-11">scroll container</a> is guaranteed to rest on a snap position when there are no active scrolling operations. That is, it must come to rest on a snap position at the termination of a scroll, if a valid, reachable snap position exists. If no valid, reachable snap position exists then no snapping occurs.
<p>If the content changes such that the visual viewport would no longer rest on a snap position (e.g. content is added, moved, deleted, resized), the scroll offset must be modified to maintain alignment of the visual viewport with a snap position. If the visual viewport was already aligned with a snap position prior to the content change and that snap position still exists (e.g. its associated element was not deleted), that same snap position must be satisfied after the content change.</p>
<dt><dfn class="css" data-dfn-for="scroll-snap-type" data-dfn-type="value" data-export="" id="valdef-scroll-snap-type-proximity">proximity<a class="self-link" href="#valdef-scroll-snap-type-proximity"></a></dfn>
<dd>
The visual viewport of this <a data-link-type="dfn" href="#scroll-container" id="ref-for-scroll-container-12">scroll container</a> may come to rest on a snap position at the termination of a scroll at the discretion of the UA given the parameters of the scroll.
<p>When the visual viewport currently rests on a snap position, if the content changes such that the visual viewport would no longer rest on that snap position (e.g. content is added, moved, deleted, resized), the scroll offset must be modified to maintain alignment of the visual viewport with a snap position. If the visual viewport was already aligned with a snap position prior to the content change and that snap position still exists (e.g. its associated element was not deleted) and if the user agent elects to modify the scroll offset to maintain snap position alignment, that same snap position must be satisfied after the content change.</p>
</dl>
<h3 class="heading settled" data-level="5.2" id="scroll-snap-padding"><span class="secno">5.2. </span><span class="content">Scroll Snap Padding: the <a class="property" data-link-type="propdesc" href="#propdef-scroll-snap-padding" id="ref-for-propdef-scroll-snap-padding-3">scroll-snap-padding</a> property</span><a class="self-link" href="#scroll-snap-padding"></a></h3>
<p>The <a class="property" data-link-type="propdesc" href="#propdef-scroll-snap-padding" id="ref-for-propdef-scroll-snap-padding-4">scroll-snap-padding</a> property defines the <a data-link-type="dfn" href="#snap-alignment-container" id="ref-for-snap-alignment-container-2">snap alignment container</a>, a region inset from the visual viewport of a <a data-link-type="dfn" href="#scroll-container" id="ref-for-scroll-container-13">scroll container</a> used in calculating its <a data-link-type="dfn" href="#scroll-snap-position" id="ref-for-scroll-snap-position-7">snap positions</a>. The snap alignment container is used as the alignment container when calculating valid <a data-link-type="dfn" href="#scroll-snap-position" id="ref-for-scroll-snap-position-8">snap positions</a>.</p>
<table class="def propdef" data-link-for-hint="scroll-snap-padding">
<tbody>
<tr>
<th>Name:
<td><dfn class="dfn-paneled css" data-dfn-type="property" data-export="" data-lt="scroll-snap-padding" id="propdef-scroll-snap-padding">scroll-snap-padding<span class="dfn-panel" data-deco=""><b><a href="#propdef-scroll-snap-padding">#propdef-scroll-snap-padding</a></b><b>Referenced in:</b><span><a href="#ref-for-propdef-scroll-snap-padding-1">3. Overview</a></span><span><a href="#ref-for-propdef-scroll-snap-padding-2">4. Definitions</a></span><span><a href="#ref-for-propdef-scroll-snap-padding-3">5.2. Scroll Snap Padding: the scroll-snap-padding property</a> <a href="#ref-for-propdef-scroll-snap-padding-4">(2)</a></span><span><a href="#ref-for-propdef-scroll-snap-padding-5">6.2. Scroll Snap Alignment: the scroll-snap-align property</a> <a href="#ref-for-propdef-scroll-snap-padding-6">(2)</a> <a href="#ref-for-propdef-scroll-snap-padding-7">(3)</a></span></span></dfn>
<tr class="value">
<th>Value:
<td class="prod">[ <a class="production css" data-link-type="type" href="https://www.w3.org/TR/css3-values/#length-value"><length></a> <a data-link-type="grammar" href="https://www.w3.org/TR/css3-values/#comb-one">|</a> <a class="production css" data-link-type="type" href="https://www.w3.org/TR/css3-values/#percentage-value"><percentage></a> ]<a data-link-type="grammar" href="https://www.w3.org/TR/css3-values/#mult-num-range">{1,4}</a>
<tr>
<th>Initial:
<td>0
<tr>
<th>Applies to:
<td>scroll containers
<tr>
<th>Inherited:
<td>no
<tr>
<th>Percentages:
<td>relative to the scroll container’s visual viewport
<tr>
<th>Media:
<td>interactive
<tr>
<th>Computed value:
<td>as specified, with lengths made absolute
<tr>
<th>Animatable:
<td>yes
</table>
<dl>
<dt><dfn class="css" data-dfn-for="scroll-snap-padding" data-dfn-type="value" data-export="" id="valdef-scroll-snap-padding--length--percentage-1-4">[ <a class="production css" data-link-type="type" href="https://www.w3.org/TR/css3-values/#length-value"><length></a> | <a class="production css" data-link-type="type" href="https://www.w3.org/TR/css3-values/#percentage-value"><percentage></a> ]{1,4}<a class="self-link" href="#valdef-scroll-snap-padding--length--percentage-1-4"></a></dfn>
<dd> Specifies the region inset from the visual viewport. Values are interpreted as for <a class="property" data-link-type="propdesc" href="https://www.w3.org/TR/CSS21/box.html#propdef-padding">padding</a>, and specify inward offsets from each edge of the visual viewport.
</dl>
<p>This property is a shorthand property that sets all of the <a class="property" data-link-type="propdesc">scroll-snap-padding-*</a> longhands in one declaration.</p>
<h2 class="heading settled" data-level="6" id="properties-on-the-elements"><span class="secno">6. </span><span class="content">Properties on the elements</span><a class="self-link" href="#properties-on-the-elements"></a></h2>
<h3 class="heading settled" data-level="6.1" id="scroll-snap-margin0"><span class="secno">6.1. </span><span class="content">Scroll Snap Margin: the <a class="property" data-link-type="propdesc" href="#propdef-scroll-snap-margin" id="ref-for-propdef-scroll-snap-margin-3">scroll-snap-margin</a> property</span><a class="self-link" href="#scroll-snap-margin0"></a></h3>
<p>The <a class="property" data-link-type="propdesc" href="#propdef-scroll-snap-margin" id="ref-for-propdef-scroll-snap-margin-4">scroll-snap-margin</a> property defines the <a data-link-type="dfn" href="#scroll-snap-margin" id="ref-for-scroll-snap-margin-2">scroll snap margin</a> on elements within a <a data-link-type="dfn" href="#scroll-container" id="ref-for-scroll-container-14">scroll container</a>, used in calculating <a data-link-type="dfn" href="#scroll-snap-position" id="ref-for-scroll-snap-position-9">snap positions</a> for that scroll container.</p>
<table class="def propdef" data-link-for-hint="scroll-snap-margin">
<tbody>
<tr>
<th>Name:
<td><dfn class="dfn-paneled css" data-dfn-type="property" data-export="" data-lt="scroll-snap-margin" id="propdef-scroll-snap-margin">scroll-snap-margin<span class="dfn-panel" data-deco=""><b><a href="#propdef-scroll-snap-margin">#propdef-scroll-snap-margin</a></b><b>Referenced in:</b><span><a href="#ref-for-propdef-scroll-snap-margin-1">3. Overview</a></span><span><a href="#ref-for-propdef-scroll-snap-margin-2">4. Definitions</a></span><span><a href="#ref-for-propdef-scroll-snap-margin-3">6.1. Scroll Snap Margin: the scroll-snap-margin property</a> <a href="#ref-for-propdef-scroll-snap-margin-4">(2)</a></span></span></dfn>
<tr class="value">
<th>Value:
<td class="prod"><a class="production css" data-link-type="type" href="https://www.w3.org/TR/css3-values/#length-value"><length></a><a data-link-type="grammar" href="https://www.w3.org/TR/css3-values/#mult-num-range">{1,4}</a>
<tr>
<th>Initial:
<td>0
<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>no
<tr>
<th>Percentages:
<td>n/a
<tr>
<th>Media:
<td>interactive
<tr>
<th>Computed value:
<td>as specified, with lengths made absolute
<tr>
<th>Animatable:
<td>yes
</table>
<dl>
<dt><dfn class="css" data-dfn-for="scroll-snap-margin" data-dfn-type="value" data-export="" id="valdef-scroll-snap-margin-length1-4"><a class="production css" data-link-type="type" href="https://www.w3.org/TR/css3-values/#length-value"><length></a>{1,4}<a class="self-link" href="#valdef-scroll-snap-margin-length1-4"></a></dfn>
<dd>
Specifies the outset of the element’s snap margin from the axis-aligned bounding box of the transformed border box, in the <a data-link-type="dfn" href="#scroll-container" id="ref-for-scroll-container-15">scroll container’s</a> coordinate space. Outsets are applied to this bounding box, not the border box.
<p class="note" role="note">Note: This ensures that the <a data-link-type="dfn" href="#scroll-snap-margin" id="ref-for-scroll-snap-margin-3">scroll snap margin</a> is always rectangular and axis-aligned to the <a data-link-type="dfn" href="#scroll-container" id="ref-for-scroll-container-16">scroll container’s</a> coordinate space.</p>
</dl>
<p>This property is a shorthand property that sets all of the <a class="property" data-link-type="propdesc">scroll-snap-margin-*</a> longhands in one declaration.</p>
<h3 class="heading settled" data-level="6.2" id="scroll-snap-align"><span class="secno">6.2. </span><span class="content">Scroll Snap Alignment: the <a class="property" data-link-type="propdesc" href="#propdef-scroll-snap-align" id="ref-for-propdef-scroll-snap-align-2">scroll-snap-align</a> property</span><a class="self-link" href="#scroll-snap-align"></a></h3>
<p>The <a class="property" data-link-type="propdesc" href="#propdef-scroll-snap-align" id="ref-for-propdef-scroll-snap-align-3">scroll-snap-align</a> property specifies how an element’s scroll snap margin should align with its ancestor scroll container’s snap alignment container. The two values specify the snapping behavior in the x and y axes, respectively. If only one value is specified, the second value defaults to the same value.</p>
<p class="issue" id="issue-f7391e31"><a class="self-link" href="#issue-f7391e31"></a> Should this be x/y axes, or inline/block? Starting with x/y axes for consistency with padding/area, otherwise a writing mode change would result in a axis mismatch (since padding is physical by default).</p>
<table class="def propdef" data-link-for-hint="scroll-snap-align">
<tbody>
<tr>
<th>Name:
<td><dfn class="dfn-paneled css" data-dfn-type="property" data-export="" data-lt="scroll-snap-align" id="propdef-scroll-snap-align">scroll-snap-align<span class="dfn-panel" data-deco=""><b><a href="#propdef-scroll-snap-align">#propdef-scroll-snap-align</a></b><b>Referenced in:</b><span><a href="#ref-for-propdef-scroll-snap-align-1">3. Overview</a></span><span><a href="#ref-for-propdef-scroll-snap-align-2">6.2. Scroll Snap Alignment: the scroll-snap-align property</a> <a href="#ref-for-propdef-scroll-snap-align-3">(2)</a></span></span></dfn>
<tr class="value">
<th>Value:
<td class="prod">[ none <a data-link-type="grammar" href="https://www.w3.org/TR/css3-values/#comb-one">|</a> start <a data-link-type="grammar" href="https://www.w3.org/TR/css3-values/#comb-one">|</a> end <a data-link-type="grammar" href="https://www.w3.org/TR/css3-values/#comb-one">|</a> center ]<a data-link-type="grammar" href="https://www.w3.org/TR/css3-values/#mult-num-range">{1,2}</a>
<tr>
<th>Initial:
<td>none
<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>no
<tr>
<th>Percentages:
<td>n/a
<tr>
<th>Media:
<td>interactive
<tr>
<th>Computed value:
<td>two keywords
<tr>
<th>Animatable:
<td>no
</table>
<dl>
<dt><dfn class="css" data-dfn-for="scroll-snap-align" data-dfn-type="value" data-export="" id="valdef-scroll-snap-align-none">none<a class="self-link" href="#valdef-scroll-snap-align-none"></a></dfn>
<dd> This box does not define a <a data-link-type="dfn" href="#scroll-snap-position" id="ref-for-scroll-snap-position-10">snap position</a> in the specified axis.
<dt><dfn class="css" data-dfn-for="scroll-snap-align" data-dfn-type="value" data-export="" id="valdef-scroll-snap-align-start">start<a class="self-link" href="#valdef-scroll-snap-align-start"></a></dfn>
<dd> The scroll offset which aligns the start edge of this box’s scroll snap margin with the start edge of its ancestor scroll container’s region defined by <a class="property" data-link-type="propdesc" href="#propdef-scroll-snap-padding" id="ref-for-propdef-scroll-snap-padding-5">scroll-snap-padding</a> in the specified axis is a <a data-link-type="dfn" href="#scroll-snap-position" id="ref-for-scroll-snap-position-11">snap position</a> in that axis.
<dt><dfn class="css" data-dfn-for="scroll-snap-align" data-dfn-type="value" data-export="" id="valdef-scroll-snap-align-end">end<a class="self-link" href="#valdef-scroll-snap-align-end"></a></dfn>
<dd> The scroll offset which aligns the end edge of this box’s scroll snap margin with the end edge of its ancestor scroll container’s region defined by <a class="property" data-link-type="propdesc" href="#propdef-scroll-snap-padding" id="ref-for-propdef-scroll-snap-padding-6">scroll-snap-padding</a> in the specified axis is a <a data-link-type="dfn" href="#scroll-snap-position" id="ref-for-scroll-snap-position-12">snap position</a> in that axis.
<dt><dfn class="css" data-dfn-for="scroll-snap-align" data-dfn-type="value" data-export="" id="valdef-scroll-snap-align-center">center<a class="self-link" href="#valdef-scroll-snap-align-center"></a></dfn>
<dd> The scroll offset which aligns the center of this box’s scroll snap margin with the center of its ancestor scroll container’s region defined by <a class="property" data-link-type="propdesc" href="#propdef-scroll-snap-padding" id="ref-for-propdef-scroll-snap-padding-7">scroll-snap-padding</a> in the specified axis is a <a data-link-type="dfn" href="#scroll-snap-position" id="ref-for-scroll-snap-position-13">snap position</a> in that axis.
</dl>
<h2 class="heading settled" data-level="7" id="acknowledgements"><span class="secno">7. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
<p>Many thanks to lots of people for their proposals and recommendations, some of which are incorporated into this document.</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. <
7DD6
/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-scroll-snap-align-center">center</a><span>, in §6.2</span>
<li><a href="#valdef-scroll-snap-align-end">end</a><span>, in §6.2</span>
<li><a href="#valdef-scroll-snap-padding--length--percentage-1-4">[ <length></a><span>, in §5.2</span>
<li><a href="#valdef-scroll-snap-margin-length1-4"><length>{1,4}</a><span>, in §6.1</span>
<li><a href="#valdef-scroll-snap-type-mandatory">mandatory</a><span>, in §5.1</span>
<li>
none
<ul>
<li><a href="#valdef-scroll-snap-type-none">value for scroll-snap-type</a><span>, in §5.1</span>
<li><a href="#valdef-scroll-snap-align-none">value for scroll-snap-align</a><span>, in §6.2</span>
</ul>
<li><a href="#valdef-scroll-snap-padding--length--percentage-1-4"><percentage> ]{1,4}</a><span>, in §5.2</span>
<li><a href="#valdef-scroll-snap-type-proximity">proximity</a><span>, in §5.1</span>
<li><a href="#scroll-container">scroll container</a><span>, in §4</span>
<li><a href="#propdef-scroll-snap-align">scroll-snap-align</a><span>, in §6.2</span>
<li><a href="#propdef-scroll-snap-margin">scroll-snap-margin</a><span>, in §6.1</span>
<li><a href="#scroll-snap-margin">scroll snap margin</a><span>, in §4</span>
<li><a href="#propdef-scroll-snap-padding">scroll-snap-padding</a><span>, in §5.2</span>
<li><a href="#scroll-snap-position">scroll snap position</a><span>, in §4</span>
<li><a href="#propdef-scroll-snap-type">scroll-snap-type</a><span>, in §5.1</span>
<li><a href="#snap-alignment-container">snap alignment container</a><span>, in §4</span>
<li><a href="#scroll-snap-margin">snap margin</a><span>, in §4</span>
<li><a href="#scroll-snap-position">snap position</a><span>, in §4</span>
<li><a href="#valdef-scroll-snap-align-start">start</a><span>, in §6.2</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">[css-align-3]</a> defines the following terms:
<ul>
<li><a href="https://www.w3.org/TR/css3-align/#alignment-container">alignment container</a>
<li><a href="https://www.w3.org/TR/css3-align/#alignment-subject">alignment subject</a>
</ul>
<li>
<a data-link-type="biblio">[css-display-3]</a> defines the following terms:
<ul>
<li><a href="https://drafts.csswg.org/css-display-3/#containing-block-chain">containing block chain</a>
</ul>
<li>
<a data-link-type="biblio">[css-pseudo-4]</a> defines the following terms:
<ul>
<li><a href="https://drafts.csswg.org/css-pseudo-4/#selectordef-first-letter">::first-letter</a>
<li><a href="https://www.w3.org/TR/css-pseudo-4/#selectordef-first-line">::first-line</a>
</ul>
<li>
<a data-link-type="biblio">[CSS3VAL]</a> defines the following terms:
<ul>
<li><a href="https://www.w3.org/TR/css3-values/#length-value"><length></a>
<li><a href="https://www.w3.org/TR/css3-values/#percentage-value"><percentage></a>
<li><a href="https://www.w3.org/TR/css3-values/#mult-num-range">{a,b}</a>
<li><a href="https://www.w3.org/TR/css3-values/#comb-one">|</a>
</ul>
<li>
<a data-link-type="biblio">[CSS21]</a> defines the following terms:
<ul>
<li><a href="https://www.w3.org/TR/CSS21/box.html#propdef-padding">padding</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-align-3">[CSS-ALIGN-3]
<dd>Elika Etemad; Tab Atkins Jr.. <a href="http://dev.w3.org/csswg/css-align/">CSS Box Alignment Module Level 3</a>. 18 December 2014. WD. URL: <a href="http://dev.w3.org/csswg/css-align/">http://dev.w3.org/csswg/css-align/</a>
<dt id="biblio-css-display-3">[CSS-DISPLAY-3]
<dd>Tab Atkins Jr.; Elika Etemad. <a href="http://dev.w3.org/csswg/css-display/">CSS Display Module Level 3</a>. 15 October 2015. WD. URL: <a href="http://dev.w3.org/csswg/css-display/">http://dev.w3.org/csswg/css-display/</a>
<dt id="biblio-css-pseudo-4">[CSS-PSEUDO-4]
<dd>Daniel Glazman; Elika Etemad; Alan Stearns. <a href="http://dev.w3.org/csswg/css-pseudo-4/">CSS Pseudo-Elements Module Level 4</a>. 15 January 2015. WD. URL: <a href="http://dev.w3.org/csswg/css-pseudo-4/">http://dev.w3.org/csswg/css-pseudo-4/</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-css3val">[CSS3VAL]
<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-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>
<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">Computed value
<tbody>
<tr>
<th scope="row"><a class="css" data-link-type="property" href="#propdef-scroll-snap-type">scroll-snap-type</a>
<td>none | mandatory | proximity
<td>none
<td>all elements
<td>no
<td>n/a
<td>interactive
<td>no
<td>as specified
<tr>
<th scope="row"><a class="css" data-link-type="property" href="#propdef-scroll-snap-padding">scroll-snap-padding</a>
<td>[ <length> | <percentage> ]{1,4}
<td>0
<td>scroll containers
<td>no
<td>relative to the scroll container’s visual viewport
<td>interactive
<td>yes
<td>as specified, with lengths made absolute
<tr>
<th scope="row"><a class="css" data-link-type="property" href="#propdef-scroll-snap-margin">scroll-snap-margin</a>
<td><length>{1,4}
<td>0
<td>all elements
<td>no
<td>n/a
<td>interactive
<td>yes
<td>as specified, with lengths made absolute
<tr>
<th scope="row"><a class="css" data-link-type="property" href="#propdef-scroll-snap-align">scroll-snap-align</a>
<td>[ none | start | end | center ]{1,2}
<td>none
<td>all elements
<td>no
<td>n/a
<td>interactive
<td>no
<td>two keywords
</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"> <b>This spec is currently undergoing major changes in line with the resolutions in <a href="https://lists.w3.org/Archives/Public/www-style/2015Dec/0048.html">https://lists.w3.org/Archives/Public/www-style/2015Dec/0048.html</a>. During this process it is not safe to assume that the current form will be representative of what it will look like in the future. After the work in process is completed, it will closely resemble <a href="https://drafts.csswg.org/css-scroll-snap/">this proposal</a>.</b><a href="#issue-752e30a5"> ↵ </a></div>
<div class="issue"> Better name for this concept?<a href="#issue-1b6eda6b"> ↵ </a></div>
<div class="issue"> <a href="https://lists.w3.org/Archives/Public/www-style/2015Nov/0328.html">Open issue</a> on whether to enhance the scroll-snap-type property for specifying the axis or adding a second property. We have <a href="https://lists.w3.org/Archives/Public/www-style/2015Dec/0048.html">resolved</a> that this functionality be added once the issue is resolved.<a href="#issue-7e0cf181"> ↵ </a></div>
<div class="issue"> Currently considering renaming the values<a href="#issue-780b32d8"> ↵ </a></div>
<div class="issue"> Should this be x/y axes, or inline/block? Starting with x/y axes for consistency with padding/area, otherwise a writing mode change would result in a axis mismatch (since padding is physical by default).<a href="#issue-f7391e31"> ↵ </a></div>
</div>
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