csswg-drafts/css-snappoints/Overview.html at 8bb64af5e09d2858117f6f7f884fce1192e0d696 · w3c/csswg-drafts · GitHub

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<!DOCTYPE html><html lang=en><head>
  <meta content="text/html; charset=utf-8" http-equiv=Content-Type>
  <title>CSS Scroll Snap Points Module Level 1</title>
  <link href=../default.css rel=stylesheet type=text/css>
  <link href=../csslogo.ico rel="shortcut icon" type=image/x-icon>
  <style>
  body {
    background: url("https://www.w3.org/StyleSheets/TR/logo-ED") top left no-repeat white;
    background-attachment: fixed;
    color: black;
    font-family: sans-serif;
    margin: 0 auto;
    max-width: 50em;
    padding: 2em 1em 2em 70px;
  }
  :link { color: #00C; background: transparent }
  :visited { color: #609; background: transparent }
  a[href]:active { color: #C00; background: transparent }
  a[href]:hover { background: #ffa }

  a[href] img { border-style: none }

  h1, h2, h3, h4, h5, h6 { text-align: left }
  h1, h2, h3 { color: #005A9C; }
  h1 { font: 170% sans-serif }
  h2 { font: 140% sans-serif }
  h3 { font: 120% sans-serif }
  h4 { font: bold 100% sans-serif }
  h5 { font: italic 100% sans-serif }
  h6 { font: small-caps 100% sans-serif }

  .hide { display: none }

  div.head { margin-bottom: 1em }
  div.head h1 { margin-top: 2em; clear: both }
  div.head table { margin-left: 2em; margin-top: 2em }

  p.copyright { font-size: small }
  p.copyright small { font-size: small }

  pre { margin-left: 2em }
  dt { font-weight: bold }

  ul.toc, ol.toc {
    list-style: none;
  }
  </style>
</head>
<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 Scroll Snap Points Module Level 1</h1>
  <h2 class="no-num no-toc no-ref heading settled" id=subtitle><span class=content>Editor’s Draft,
    <span class=dt-updated><span class=value-title title=20141106>6 November 2014</span></span></span></h2>
  <div data-fill-with=spec-metadata><dl><dt>This version:<dd><a class=u-url href=http://dev.w3.org/csswg/css-snappoints/>http://dev.w3.org/csswg/css-snappoints/</a><dt>Feedback:<dd><span><a href="mailto:www-style@w3.org?subject=[css-snappoints] feedback">www-style@w3.org</a> with subject line “<kbd>[css-snappoints] <var>… message topic …</var></kbd>” (<a href=http://lists.w3.org/Archives/Public/www-style/ rel=discussion>archives</a>)</span><dt>Editors:<dd><div class="p-author h-card vcard"><span class="p-name fn">Matt Rakow</span> (<span class="p-org org">Microsoft</span>)</div><dd><div class="p-author h-card vcard"><span class="p-name fn">Jacob Rossi</span> (<span class="p-org org">Microsoft</span>)</div><dt>Issue Tracking:<dd><span><a href=http://wiki.csswg.org/spec/css-snappoints>http://wiki.csswg.org/spec/css-snappoints</a></span></dl></div>
  <div data-fill-with=warning></div>
</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 points".</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>
	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-snappoints%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-snappoints” in the subject,
	preferably like this:
	“[css-snappoints] <em>…summary of comment…</em>”

<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>
	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>
	This document is governed by the <a href=http://www.w3.org/2014/Process-20140801/>1 August 2014 W3C Process Document</a>.
</div>
<div data-fill-with=at-risk></div>

<h2 class="no-num no-toc no-ref heading settled" id=contents><span class=content>Table of Contents</span></h2>
<div data-fill-with=table-of-contents><ul class=toc><li><a href=#intro><span class=secno>1</span> <span class=content>Introduction</span></a><ul 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></ul><li><a href=#examples><span class=secno>2</span> <span class=content>Motivating Examples</span></a><li><a href=#definitions><span class=secno>3</span> <span class=content>Definitions</span></a><li><a href=#scroll-snap-type><span class=secno>4</span> <span class=content>Scroll Snap Types: the <span class=property data-link-type=propdesc title=scroll-snap-type>scroll-snap-type</span> property</span></a><li><a href=#scroll-snap-points><span class=secno>5</span> <span class=content>Scroll Snap Points: the <span class=property data-link-type=propdesc title=scroll-snap-points-x>scroll-snap-points-x</span> and <span class=property data-link-type=propdesc title=scroll-snap-points-y>scroll-snap-points-y</span> properties</span></a><li><a href=#scroll-snap-destination><span class=secno>6</span> <span class=content>Scroll Snap Destination: the <span class=property data-link-type=propdesc title=scroll-snap-destination>scroll-snap-destination</span> property</span></a><li><a href=#scroll-snap-coordinate><span class=secno>7</span> <span class=content>Scroll Snap Coordinate: the <span class=property data-link-type=propdesc title=scroll-snap-coordinate>scroll-snap-coordinate</span> property</span></a><li><a href=#acknowledgments><span class=secno></span> <span class=content>Acknowledgments</span></a><li><a href=#conformance><span class=secno></span> <span class=content>
Conformance</span></a><ul class=toc><li><a href=#conventions><span class=secno></span> <span class=content>
Document conventions</span></a><li><a href=#conformance-classes><span class=secno></span> <span class=content>
Conformance classes</span></a><li><a href=#partial><span class=secno></span> <span class=content>
Partial implementations</span></a><li><a href=#experimental><span class=secno></span> <span class=content>
Experimental implementations</span></a><li><a href=#testing><span class=secno></span> <span class=content>
Non-experimental implementations</span></a></ul><li><a href=#references><span class=secno></span> <span class=content>References</span></a><ul 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></ul><li><a href=#index><span class=secno></span> <span class=content>Index</span></a><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></ul></div>


<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 points
    which enforce the scroll offsets that a <a data-link-type=dfn href=#scroll-container title="scroll container’s">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-biblio-type=normative data-link-type=biblio href=#biblio-css21 title=CSS21>[CSS21]</a> section 11.1.</p>

<p>None of the properties in this module apply to the <span class=css>::first-line</span> and <span class=css>::first-letter</span> 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=http://www.w3.org/TR/CSS21/about.html#property-defs>CSS property
    definition conventions</a> from <a data-biblio-type=normative data-link-type=biblio href=#biblio-css21 title=CSS21>[CSS21]</a>. Value types not defined in
    this specification are defined in CSS Level 2 Revision 1 <a data-biblio-type=normative data-link-type=biblio href=#biblio-css21 title=CSS21>[CSS21]</a>.
    Other CSS modules may expand the definitions of these value types: for
    example <a data-biblio-type=informative data-link-type=biblio href=#biblio-css3val title=CSS3VAL>[CSS3VAL]</a>, when combined with this module, expands the
    definition of the <a class="
8DD2
;production css" data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#length-value title="<length>">&lt;length&gt;</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>
        In this example, a series of images arranged in a <a data-link-type=dfn href=#scroll-container title="scroll container">scroll container</a>
        are used to build a photo gallery.
        The <a data-link-type=dfn href=#scroll-container title="scroll container">scroll container</a> is sized to the same size of the images contained therein.
        Using mandatory snap points,
        scrolling will always complete with a snap point aligned to the edge of the <a data-link-type=dfn href=#scroll-container title="scroll container’s">scroll container’s</a> visual viewport.
        By aligning a snap point at the edge of each image,
        this creates a photo viewer were users can scroll through the images one at a time.

        <pre>img {
    width:500px;
}
.photoGallery {
    width: 500px;
    overflow-x: auto;
    overflow-y: hidden;
    white-space: nowrap;
    /* Sets up points to which scrolling
       will snap along x-axis */
    scroll-snap-points-x: repeat(100%);
    /* Requires that scrolling always end at a snap point when
       the operation completes (hard snap) */
    scroll-snap-type: mandatory;
}
</pre>

        <pre>&lt;div class="photoGallery"&gt;
    &lt;img src="img1.jpg"&gt;
    &lt;img src="img2.jpg"&gt;
    &lt;img src="img3.jpg"&gt;
    &lt;img src="img4.jpg"&gt;
    &lt;img src="img5.jpg"&gt;
&lt;/div&gt;
</pre>

        <figure>
            <img alt="" src=interval_snap_points.png>

            <figcaption>
                The layout of the scroll container’s contents in the example.
                Snap points are set along the x-axis,
                starting at 0px and repeating at intervals of 100% of the scroll container’s width.
            </figcaption>
        </figure>
    </div>

    <div class=example>
        This example also builds a photo gallery as in example 1.  However, in this example the <a data-link-type=dfn href=#scroll-container title="scroll container">scroll container</a>
        is larger than the photos contained within (such that multiple images may be seen simultaneously), and the image
        sizes vary (such that a repeating snap interval would not be effective).  Using mandatory element-based snap
        points, scrolling will always complete with an image centered in the <a data-link-type=dfn href=#scroll-container title="scroll container’s">scroll container’s</a> visual viewport.

        <pre>img {
    /* Defines the center of each photo as the
       coordinate that should be used for snapping */
    scroll-snap-coordinate: 50% 50%;
}
.photoGallery {
    width: 500px;
    overflow-x: auto;
    overflow-y: hidden;
    white-space: nowrap;
    /* Specifies that each element’s snap coordinate should
       align with the center of the scroll container */
    scroll-snap-destination: 50% 50%;
    /* Requires that scrolling always end at a snap point
       when the operation completes (hard snap). */
    scroll-snap-type: mandatory;
}
</pre>

        <pre>&lt;div class="photoGallery"&gt;
    &lt;img src="img1.jpg"&gt;
    &lt;img src="img2.jpg"&gt;
    &lt;img src="img3.jpg"&gt;
    &lt;img src="img4.jpg"&gt;
    &lt;img src="img5.jpg"&gt;
&lt;/div&gt;
</pre>

        <figure>
            <img alt="" src=element_snap_points.png>

            <figcaption>
                The layout of the scroll container’s contents in the example.
                The snap-destination is horizontally and vertically centered within the scroll container’s visual viewport
                (represented by a red X),
                and each element has its snap coordinate horizontally and vertically centered within the element
                (represented by yellow plus signs).
            </figcaption>
        </figure>
    </div>

    <div class=example>
        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 title="scroll container">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 points instead of mandatory snap points 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 point, then the scroll will be adjusted to
        align the page as specified.

        <pre>.page {
    /* Defines the top center of each page as the
       coordinate that should be used for snapping */
    scroll-snap-coordinate: 50% 0;
}
.docScroller {
    width: 500px;
    overflow-x: hidden;
    overflow-y: auto;
    /* Specifies that each element’s snap coordinate should
       align with the center of the scroll container, offset
       a short distance from the top edge. */
    scroll-snap-destination: 50% 100px;
     /* Encourages scrolling to end at a snap point when the
        operation completes, if it is near a snap point */
    scroll-snap-type: proximity;
}
</pre>

        <pre>&lt;div class="docScroller"&gt;
    &lt;div class="page"&gt;Page 1&lt;/div&gt;
    &lt;div class="page"&gt;Page 2&lt;/div&gt;
    &lt;div class="page"&gt;Page 3&lt;/div&gt;
    &lt;div class="page"&gt;Page 4&lt;/div&gt;
&lt;/div&gt;
</pre>

        <figure>
            <img alt="" src=element_snap_points_offset.png>

            <figcaption>
                The layout of the scroll container’s contents in the example.
                The snap-destination is horizontally centered and offset 100px from the top edge with respect to the scroll container’s visual viewport
                (represented by a red X),
                and each element has its snap coordinate horizontally centered and top-aligned with respect to the element
                (represented by yellow plus signs).
            </figcaption>
        </figure>
    </div>

<h2 class="heading settled" data-level=3 id=definitions><span class=secno>3. </span><span class=content>Definitions</span><a class=self-link href=#definitions></a></h2>

    <dl>
        <dt><dfn data-dfn-type=dfn data-export="" id=scroll-container>scroll container<a class=self-link href=#scroll-container></a></dfn>
        <dd>
            An element which provides a scrolling user interface as described in <a data-biblio-type=normative data-link-type=biblio href=#biblio-css21 title=CSS21>[CSS21]</a>, particularly in the section on overflow.
    </dl>

<h2 class="heading settled" data-level=4 id=scroll-snap-type><span class=secno>4. </span><span class=content>Scroll Snap Types: the <a class=property data-link-type=propdesc href=#propdef-scroll-snap-type title=scroll-snap-type>scroll-snap-type</a> property</span><a class=self-link href=#scroll-snap-type></a></h2>

<p>The <span class=css>scroll-snap-type</span> property is used to define how strictly snap points are enforced on the <a data-link-type=dfn href=#scroll-container title="scroll container">scroll container</a>, if any are present. It defines how and when snap points are enforced on the visual viewport of the scroll container it is applied to in order to adjust scroll offset.  It intentionally does not specify nor mandate any precise animations or physics used to enforce those snap points; this is left up to the user agent.</p>

<table class="definition propdef"><tr><th>Name:<td><dfn class=css data-dfn-type=property data-export="" id=propdef-scroll-snap-type>scroll-snap-type<a class=self-link href=#propdef-scroll-snap-type></a></dfn><tr><th>Value:<td class=prod>none <a data-link-for="" data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one title=|>|</a> mandatory <a data-link-for="" data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one title=|>|</a> proximity<tr><th>Initial:<td>none<tr><th>Applies to:<td><a data-link-type=dfn href=#scroll-container title="scroll containers">scroll containers</a><tr><th>Inherited:<td>no<tr><th>Percentages:<td>n/a<tr><th>Media:<td>interactive<tr><th>Computed value:<td>specified value<tr><th>Animatable:<td>no</table>

    <dl data-dfn-for=scroll-snap-type data-dfn-type=value>
        <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 title="scroll container">scroll container</a> must ignore snap points, if any, when scrolled.

        <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 title="scroll container">scroll container</a> is guaranteed to rest on a snap point when there are no active scrolling operations.  That is, it must come to rest on a snap point at the termination of a scroll, if possible.  If the content changes such that the visual viewport would no longer rest on a snap point (e.g. content is added, moved, deleted, resized), the scroll offset must be modified to maintain this guarantee.

        <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 title="scroll container">scroll container</a> may come to rest on a snap point at the termination of a scroll at the discretion of the UA given the parameters of the scroll.  If the content changes such that the visual viewport would no longer rest on a snap point (e.g. content is added, moved, deleted, resized), the scroll offset may be modified to maintain this guarantee.
    </dl>

    <div class=issue id=issue-b2e1d9bc><a class=self-link href=#issue-b2e1d9bc></a>
        Describe the guarantee as an invariant for better clarity.  Include edge case behavior such as mandatory snap points when there is no satisfiable snap point.
    </div>

<h2 class="heading settled" data-level=5 id=scroll-snap-points><span class=secno>5. </span><span class=content>Scroll Snap Points: the <a class=property data-link-type=propdesc href=#propdef-scroll-snap-points-x title=scroll-snap-points-x>scroll-snap-points-x</a> and <a class=property data-link-type=propdesc href=#propdef-scroll-snap-points-y title=scroll-snap-points-y>scroll-snap-points-y</a> properties</span><a class=self-link href=#scroll-snap-points></a></h2>

<p>The <a class=property data-link-type=propdesc href=#propdef-scroll-snap-points-x title=scroll-snap-points-x>scroll-snap-points-x</a> and <a class=property data-link-type=propdesc href=#propdef-scroll-snap-points-y title=scroll-snap-points-y>scroll-snap-points-y</a> properties
    are used to define the positioning of snap points
    within the content of the <a data-link-type=dfn href=#scroll-container title="scroll container">scroll container</a> they are applied to.</p>

<table class="definition propdef"><tr><th>Name:<td><dfn class=css data-dfn-type=property data-export="" id=propdef-scroll-snap-points-x>scroll-snap-points-x<a class=self-link href=#propdef-scroll-snap-points-x></a></dfn>, <dfn class=css data-dfn-type=property data-export="" id=propdef-scroll-snap-points-y>scroll-snap-points-y<a class=self-link href=#propdef-scroll-snap-points-y></a></dfn><tr><th>Value:<td class=prod>none <a data-link-for="" data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one title=|>|</a> repeat(<a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#length-value title="<length>">&lt;length&gt;</a>)<tr><th>Initial:<td>none<tr><th>Applies to:<td><a data-link-type=dfn href=#scroll-container title="scroll containers">scroll containers</a><tr><th>Inherited:<td>no<tr><th>Percentages:<td>relative to same axis of the padding-box of the <a data-link-type=dfn href=#scroll-container title="scroll container">scroll container</a><tr><th>Media:<td>interactive<tr><th>Computed value:<td>specified value, with lengths made absolute<tr><th>Animatable:<td>no</table>

    <dl data-dfn-for="snap-points-x, snap-points-y" data-dfn-type=value>
        <dt><dfn class=css data-dfn-for="snap-points-x, snap-points-y" data-dfn-type=value data-export="" id=valdef-snap-points-x-snap-points-y-none>none<a class=self-link href=#valdef-snap-points-x-snap-points-y-none></a></dfn>
        <dd>
            No snap points are defined by this <a data-link-type=dfn href=#scroll-container title="scroll container">scroll container</a>.  Contained elements may still define snap points on this <a data-link-type=dfn href=#scroll-container title="scroll container’s">scroll container’s</a> behalf.

        <dt><dfn class=css data-dfn-for="snap-points-x, snap-points-y" data-dfn-type=value data-export="" id=valdef-snap-points-x-snap-points-y-repeatlength>repeat(<a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#length-value title="<length>">&lt;length&gt;</a>)<a class=self-link href=#valdef-snap-points-x-snap-points-y-repeatlength></a></dfn>
        <dd>
            Defines an interval at which snap points are defined, starting from the container’s relevant <a data-link-type=dfn href=http://dev.w3.org/csswg/css-writing-modes-3/#start title=start>start</a> edge.  Only positive lengths are allowed.
    </dl>

    <div class=issue id=issue-1011500c><a class=self-link href=#issue-1011500c></a>
        Should there be a way to specify that either end of the scrollable content should have a snap point?
    </div>

<h2 class="heading settled" data-level=6 id=scroll-snap-destination><span class=secno>6. </span><span class=content>Scroll Snap Destination: the <a class=property data-link-type=propdesc href=#propdef-scroll-snap-destination title=scroll-snap-destination>scroll-snap-destination</a> property</span><a class=self-link href=#scroll-snap-destination></a></h2>

<p>The <a class=property data-link-type=propdesc href=#propdef-scroll-snap-destination title=scroll-snap-destination>scroll-snap-destination</a> property is used to define the x and y coordinate within the <a data-link-type=dfn href=#scroll-container title="scroll container’s">scroll container’s</a> visual viewport
    which element snap points will align with.</p>

<table class="definition propdef"><tr><th>Name:<td><dfn class=css data-dfn-type=property data-export="" id=propdef-scroll-snap-destination>scroll-snap-destination<a class=self-link href=#propdef-scroll-snap-destination></a></dfn><tr><th>Value:<td class=prod><a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-backgrounds-3/#position title="<position>">&lt;position&gt;</a><tr><th>Initial:<td>0px 0px<tr><th>Applies to:<td><a data-link-type=dfn href=#scroll-container title="scroll containers">scroll containers</a><tr><th>Inherited:<td>no<tr><th>Percentages:<td>relative to width and height of the padding-box of the <a data-link-type=dfn href=#scroll-container title="scroll container">scroll container</a><tr><th>Media:<td>interactive<tr><th>Computed value:<td>specified value, with lengths made absolute<tr><th>Animatable:<td>yes</table>

    <dl data-dfn-for=scroll-snap-destination data-dfn-type=value>
        <dt><dfn class=css data-dfn-for=scroll-snap-destination data-dfn-type=value data-export="" id=valdef-scroll-snap-destination-position><a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-backgrounds-3/#position title="<position>">&lt;position&gt;</a><a class=self-link href=#valdef-scroll-snap-destination-position></a></dfn>
        <dd>
            Specifies the offset of the snap destination from the start edge of the <a data-link-type=dfn href=#scroll-container title="scroll container’s">scroll container’s</a> visual viewport.  The first value gives the x coordinate of the snap destination, the second value its y coordinate.
    </dl>

<h2 class="heading settled" data-level=7 id=scroll-snap-coordinate><span class=secno>7. </span><span class=content>Scroll Snap Coordinate: the <a class=property data-link-type=propdesc href=#propdef-scroll-snap-coordinate title=scroll-snap-coordinate>scroll-snap-coordinate</a> property</span><a class=self-link href=#scroll-snap-coordinate></a></h2>

<p>The <a class=property data-link-type=propdesc href=#propdef-scroll-snap-coordinate title=scroll-snap-coordinate>scroll-snap-coordinate</a> property is used to define the x and y coordinate within the element
    which will align with the nearest ancestor <a data-link-type=dfn href=#scroll-container title="scroll container’s">scroll container’s</a> snap-destination for the respective axis.  In the case that the element has been transformed, the snap coordinate is also transformed in the same way (such that the snap-point is aligned with the element as-drawn).</p>

    <p class=issue id=issue-02f28956
8DD2
><a class=self-link href=#issue-02f28956></a>
        How does this work with fragmentation?
    </p>

    <p class=issue id=issue-0bada396><a class=self-link href=#issue-0bada396></a>
        Consider alternative naming besides "coordinate".  Consider naming conventions like in Grid Layout for grouping properties on the container vs. items.
    </p>

<table class="definition propdef"><tr><th>Name:<td><dfn class=css data-dfn-type=property data-export="" id=propdef-scroll-snap-coordinate>scroll-snap-coordinate<a class=self-link href=#propdef-scroll-snap-coordinate></a></dfn><tr><th>Value:<td class=prod>none <a data-link-for="" data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one title=|>|</a> <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-backgrounds-3/#position title="<position>">&lt;position&gt;</a><a data-link-for="" data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#mult-comma title=#>#</a><tr><th>Initial:<td>none<tr><th>Applies to:<td>all elements<tr><th>Inherited:<td>no<tr><th>Percentages:<td>refer to the element’s border box<tr><th>Media:<td>interactive<tr><th>Computed value:<td>specified value, with lengths made absolute<tr><th>Animatable:<td>yes</table>

    <dl data-dfn-for=scroll-snap-coordinate data-dfn-type=value>
        <dt><dfn class=css data-dfn-for=scroll-snap-coordinate data-dfn-type=value data-export="" id=valdef-scroll-snap-coordinate-none>none<a class=self-link href=#valdef-scroll-snap-coordinate-none></a></dfn>
        <dd>
            Specifies that this element does not contribute a snap point.

        <dt><dfn class=css data-dfn-for=scroll-snap-coordinate data-dfn-type=value data-export="" id=valdef-scroll-snap-coordinate-position><a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-backgrounds-3/#position title="<position>">&lt;position&gt;</a>#<a class=self-link href=#valdef-scroll-snap-coordinate-position></a></dfn>
        <dd>
            Specifies the offset of the snap coordinates from the start edge of the element’s border box.  For each pairing, the first value gives the x coordinate of the snap coordinate, the second value its y coordinate.
    </dl>

    <p class=issue id=issue-cec53866><a class=self-link href=#issue-cec53866></a>
        How can an alternative box be specified?
    </p>

<h2 class="no-num heading settled" id=acknowledgments><span class=content>Acknowledgments</span><a class=self-link href=#acknowledgments></a></h2>

<p>Many thanks to lots of people for their proposals and recommendations, some of which are incorporated into this document.</p>

<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="no-ref heading settled" id=conventions><span class=content>
Document conventions</span><a class=self-link href=#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>All of the text of this specification is normative except sections
    explicitly marked as non-normative, examples, and notes. <a data-biblio-type=normative data-link-type=biblio href=#biblio-rfc2119 title=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:

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

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

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

    <p>Advisements are normative sections styled to evoke special attention and are
    set apart from other normative text with <code>&lt;strong class="advisement"&gt;</code>, like
    this:

    <strong class=advisement>
        UAs MUST provide an accessible alternative.
    </strong>

<h3 class="no-ref heading settled" id=conformance-classes><span class=content>
Conformance classes</span><a class=self-link href=#conformance-classes></a></h3>

    <p>Conformance to this specification
    is defined for three conformance classes:
    <dl>
        <dt>style sheet
            <dd>A <a href=http://www.w3.org/TR/CSS21/conform.html#style-sheet>CSS
            style sheet</a>.
        <dt>renderer
            <dd>A <a href=http://www.w3.org/TR/CSS21/conform.html#user-agent>UA</a>
            that interprets the semantics of a style sheet and renders
            documents that use them.
        <dt>authoring tool
            <dd>A <a href=http://www.w3.org/TR/CSS21/conform.html#user-agent>UA</a>
            that writes a style sheet.
    </dl>

    <p>A style sheet is conformant to 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>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>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.

<h3 class="no-ref heading settled" id=partial><span class=content>
Partial implementations</span><a class=self-link href=#partial></a></h3>

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

<h3 class="no-ref heading settled" id=experimental><span class=content>
Experimental implementations</span><a class=self-link href=#experimental></a></h3>

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

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

<h3 class="no-ref heading settled" id=testing><span class=content>
Non-experimental implementations</span><a class=self-link href=#testing></a></h3>

    <p>Once a specification reaches the Candidate Recommendation stage,
    non-experimental implementations are possible, and implementors should
    release an unprefixed implementation of any CR-level feature they
    can demonstrate to be correctly implemented according to spec.

    <p>To establish and maintain the interoperability of CSS across
    implementations, the CSS Working Group requests that non-experimental
    CSS renderers submit an implementation report (and, if necessary, the
    testcases used for that implementation report) to the W3C before
    releasing an unprefixed implementation of any CSS features. Testcases
    submitted to W3C are subject to review and correction by the CSS
    Working Group.

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


<h2 class="no-num heading settled" id=references><span class=content>References</span><a class=self-link href=#references></a></h2><h3 class="no-num heading settled" id=normative><span class=content>Normative References</span><a class=self-link href=#normative></a></h3><dl><dt id=biblio-css21 title=CSS21><a class=self-link href=#biblio-css21></a>[CSS21]<dd>Bert Bos; et al. <a href=http://www.w3.org/TR/2011/REC-CSS2-20110607>Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification</a>. 7 June 2011. W3C Recommendation. URL: <a href=http://www.w3.org/TR/2011/REC-CSS2-20110607>http://www.w3.org/TR/2011/REC-CSS2-20110607</a><dt id=biblio-rfc2119 title=rfc2119><a class=self-link href=#biblio-rfc2119></a>[rfc2119]<dd>S. Bradner. <a href=http://www.ietf.org/rfc/rfc2119.txt>Key words for use in RFCs to Indicate Requirement Levels</a>. March 1997. Best Current Practice. URL: <a href=http://www.ietf.org/rfc/rfc2119.txt>http://www.ietf.org/rfc/rfc2119.txt</a></dl><h3 class="no-num heading settled" id=informative><span class=content>Informative References</span><a class=self-link href=#informative></a></h3><dl><dt id=biblio-css3val title=CSS3VAL><a class=self-link href=#biblio-css3val></a>[CSS3VAL]<dd>Håkon Wium Lie; Tab Atkins; Elika J. Etemad. <a href=http://www.w3.org/TR/2013/CR-css3-values-20130730/>CSS Values and Units Module Level 3</a>. 30 July 2013. W3C Candidate Recommendation. (Work in progress.) URL: <a href=http://www.w3.org/TR/2013/CR-css3-values-20130730/>http://www.w3.org/TR/2013/CR-css3-values-20130730/</a></dl><h2 class="no-num heading settled" id=index><span class=content>Index</span><a class=self-link href=#index></a></h2><ul class=indexlist><li>mandatory, <a href=#valdef-scroll-snap-type-mandatory title="section 4">4</a><li>none<ul><li>value for scroll-snap-type, <a href=#valdef-scroll-snap-type-none title="section 4">4</a><li>value for snap-points-x,, snap-points-y, <a href=#valdef-snap-points-x-snap-points-y-none title="section 5">5</a><li>value for scroll-snap-coordinate, <a href=#valdef-scroll-snap-coordinate-none title="section 7">7</a></ul><li>&lt;position&gt;, <a href=#valdef-scroll-snap-destination-position title="section 6">6</a><li>&lt;position&gt;#, <a href=#valdef-scroll-snap-coordinate-position title="section 7">7</a><li>proximity, <a href=#valdef-scroll-snap-type-proximity title="section 4">4</a><li>repeat(&lt;length&gt;), <a href=#valdef-snap-points-x-snap-points-y-repeatlength title="section 5">5</a><li>scroll container, <a href=#scroll-container title="section 3">3</a><li>scroll-snap-coordinate, <a href=#propdef-scroll-snap-coordinate title="section 7">7</a><li>scroll-snap-destination, <a href=#propdef-scroll-snap-destination title="section 6">6</a><li>scroll-snap-points-x, <a href=#propdef-scroll-snap-points-x title="section 5">5</a><li>scroll-snap-points-y, <a href=#propdef-scroll-snap-points-y title="section 5">5</a><li>scroll-snap-type, <a href=#propdef-scroll-snap-type title="section 4">4</a></ul><h2 class="no-num heading settled" id=property-index><span class=content>Property Index</span><a class=self-link href=#property-index></a></h2><table class="proptable data"><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 title=scroll-snap-type>scroll-snap-type</a><td>none | mandatory | proximity<td>none<td>scroll containers<td>no<td>n/a<td>interactive<td>no<td>specified value<tr><th scope=row><a class=css data-link-type=property href=#propdef-scroll-snap-points-x title=scroll-snap-points-x>scroll-snap-points-x</a><td>none | repeat(&lt;length&gt;)<td>none<td>scroll containers<td>no<td>relative to same axis of the padding-box of the scroll container<td>interactive<td>no<td>specified value, with lengths made absolute<tr><th scope=row><a class=css data-link-type=property href=#propdef-scroll-snap-points-y title=scroll-snap-points-y>scroll-snap-points-y</a><td>none | repeat(&lt;length&gt;)<td>none<td>scroll containers<td>no<td>relative to same axis of the padding-box of the scroll container<td>interactive<td>no<td>specified value, with lengths made absolute<tr><th scope=row><a class=css data-link-type=property href=#propdef-scroll-snap-destination title=scroll-snap-destination>scroll-snap-destination</a><td>&lt;position&gt;<td>0px 0px<td>scroll containers<td>no<td>relative to width and height of the padding-box of the scroll container<td>interactive<td>yes<td>specified value, with lengths made absolute<tr><th scope=row><a class=css data-link-type=property href=#propdef-scroll-snap-coordinate title=scroll-snap-coordinate>scroll-snap-coordinate</a><td>none | &lt;position&gt;#<td>none<td>all elements<td>no<td>refer to the element’s border box<td>interactive<td>yes<td>specified value, with lengths made absolute</table><h2 class="no-num 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>
        Describe the guarantee as an invariant for better clarity.  Include edge case behavior such as mandatory snap points when there is no satisfiable snap point.
    <a href=#issue-b2e1d9bc> ↵ </a></div><div class=issue>
        Should there be a way to specify that either end of the scrollable content should have a snap point?
    <a href=#issue-1011500c> ↵ </a></div><div class=issue>
        How does this work with fragmentation?
    <a href=#issue-02f28956> ↵ </a></div><div class=issue>
        Consider alternative naming besides "coordinate".  Consider naming conventions like in Grid Layout for grouping properties on the container vs. items.
    <a href=#issue-0bada396> ↵ </a></div><div class=issue>
        How can an alternative box be specified?
    <a href=#issue-cec53866> ↵ </a></div></div>