csswg-drafts/css-snappoints/Overview.src.html at c5713de09e93d8809440c72032bc167156995baf · w3c/csswg-drafts · GitHub

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<!DOCTYPE html>
<html lang="en">
<head>
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
  <title>CSS Scroll Snap Points Module 1</title>
  <link rel=contents href="#contents">
  <link rel=index href="#index">
  <link rel="stylesheet" type="text/css" href="../default.css">
  <link href="../csslogo.ico" rel="shortcut icon" type="image/x-icon">
  <link rel="stylesheet" type="text/css" href="http://www.w3.org/StyleSheets/TR/W3C-[STATUS].css">
</head>

<body class="h-entry">

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

<h1 class="p-name">CSS Scroll Snap Points Module Level 1</h1>

<h2 class="no-num no-toc">[LONGSTATUS] <time class="dt-updated" datetime="[CDATE]">[DATE]</time> <!-- for HTML4.x doctypes: <span class="dt-updated"><span class="value-title" title="[CDATE]">[DATE]</span></span> --> </h2>
<dl>
  <dt>This version:
    <dd><a class="u-url" href="[VERSION]">[VERSION]</a>

  <dt>Latest version:
    <dd><a href="http://www.w3.org/TR/css-snappoints/">http://www.w3.org/TR/css-snappoints/</a>

  <dt>Editor's draft:
    <dd><a href="http://dev.w3.org/csswg/css-snappoints/">http://dev.w3.org/csswg/css-snappoints/</a>
    (<a href="https://dvcs.w3.org/hg/csswg/log/tip/[SHORTNAME]/Overview.src.html">change log</a>)

  <dt>Previous version:
    <dd><a href="http://www.w3.org/PreviousVersionURI">http://www.w3.org/PreviousVersionURI</a>

  <dt>Issue Tracking:</dt>
    <dd><a rel="issues" href="http://wiki.csswg.org/spec/css-snappoints"
           >http://wiki.csswg.org/spec/css-snappoints</a></dd>

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

  <dt>Editors:</dt>
    <dd class="p-author h-card vcard"><span class="p-name fn">Matt Rakow</span>, <a class="p-org org h-org" href="http://www.microsoft.com/">Microsoft Corporation</a>
    <dd class="p-author h-card vcard"><span class="p-name fn">Jacob Rossi</span>, <a class="p-org org h-org" href="http://www.microsoft.com/">Microsoft Corporation</a>

</dl>

<!--copyright-->

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

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

  <p>
  <span class="p-summary">
   <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. This module contains the features of CSS to control panning and scrolling behavior with "snap points."
  </span>


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

<!--status-->


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

<!--toc-->

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

  <p><em>This section is not normative.</em></p>

  <p>Popular UX paradigms for scrollable content frequently employ pagination of 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 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 pagination. 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 scroller may end at after a scrolling operation has completed.</p>

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

  <p>This module extends the scrolling user interface features defined in [[!CSS21]] section 11.1.

  <p>None of the properties in this module apply to the <code>::first-line</code> and
  <code>::first-letter</code> pseudo-elements.

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

  <p>This specification follows the
  <a href="http://www.w3.org/TR/CSS21/about.html#property-defs">CSS property
  definition conventions</a> from [[!CSS21]]. Value types not defined in
  this specification are defined in CSS Level 2 Revision 1 [[!CSS21]].
  Other CSS modules may expand the definitions of these value types: for
  example [[CSS3VAL]], when combined with this module, expands the
  definition of the <var>&lt;length&gt;</var> value type as used in this specification.</p>

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

<h2 id="examples">Examples
</h2>
  <div class=example>
  <p>In this example, a series of images arranged in a <span>scroller</span> are used to build a photo gallery.
      The <span>scroller</span> 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 <span>scroller</span>.
      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.</p>
  <pre>img {
    width:500px;
}
.photoGallery {
    width: 500px;
    overflow-x: auto;
    overflow-y: hidden;
    white-space: nowrap;
    scroll-snap-points-x: snapInterval(0px,100%); /*Sets up points to which scrolling will snap along x-axis */
    scroll-snap-type: mandatory; /* requires that scrolling always end at a snap point when the operation completes (hard snap) */
}</pre>
<code class="html">&lt;div class=&quot;photoGallery&quot;&gt;
    &lt;img src=&quot;img1.jpg&quot;&gt;&lt;img src=&quot;img2.jpg&quot;&gt;&lt;img src=&quot;img3.jpg&quot;&gt;&lt;img src=&quot;img4.jpg&quot;&gt;&lt;img src=&quot;img5.jpg&quot;&gt;
&lt;/div&gt;</code>

<div class=figure>
  <p><img src="example1.png"
  alt="The layout of the scroller's contents in the example. Snap points are set along the x-axis, starting at 0px and repeating at intervals of 100% of the containing block's width.">

  <p class=caption>The layout of the scroller's contents in the example. Snap points are set along
      the x-axis, starting at 0px and repeating at intervals of 100% of the containing block's width.
  </div>
  </div>
<h3 id="definitions">
Definitions</h3>
<dl>
	<dt><dfn>scroller</dfn></dt>
	<dd>An element which provides a scrolling user interface as described in [[!CSS21]], particularly in the section on overflow. </dd>
</dl>
<h3 id="scroll-snap-type">
Scroll Snap Types: the 'scroll-snap-type' property</h3>
<p>The ''scroll-snap-type'' property is used to define the physics characteristics of panning and scrolling,
    if any. It defines how and when snap points are enforced on the scroller it is applied to. </p>
  <table class=propdef>
    <tr>
      <th>Name:
      <td><dfn>scroll-snap-type</dfn>
    <tr>
      <th><a href="#values">Value</a>:
      <td>none | mandatory | proximity
    <tr>
      <th>Initial:
      <td>none
    <tr>
      <th>Applies to:
      <td>block-level <span class="index-def">scrollers</span>
    <tr>
      <th>Inherited:
      <td>no
    <tr>
      <th>Percentages:
      <td>N/A
    <tr>
      <th>Media:
      <td>interactive
    <tr>
      <th>Computed&nbsp;value:
      <td>specified value
    <tr>
      <th>Animatable:
      <td>no
    <tr>
      <th>Canonical order:
      <td><abbr title="follows order of property value definition">per grammar</abbr>
  </table>

  <dl>
    <dt><dfn>''none''</dfn></dt>
      <dd>This <span>scroller</span> <strong>must</strong> ignore snap points, if any, when scrolled.
	<dt><dfn>''mandatory''</dfn></dt>
      <dd>This <span>scroller</span> <strong>must</strong> come to rest on a snap point at the termination of a scroll, if possible.
    <dt><dfn>''proximity''</dfn></dt>
      <dd>This <span>scroller</span> <strong>may</strong> 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.
  </dl>
<h3 id="scroll-snap-points">
Scroll Snap Points: the 'scroll-snap-points-x' and 'scroll-snap-points-y properties</h3>
<p>The ''scroll-snap-points-x'' and ''scroll-snap-points-y'' properties are used to define the positioning of snap points within the content of the <span>scroller</span> they are applied to. </p>
  <table class=propdef>
    <tr>
      <th>Name:
      <td><dfn>scroll-snap-points-x</dfn>, <dfn>scroll-snap-points-y</dfn>
    <tr>
      <th><a href="#values">Value</a>:
      <td>snapInterval(<length>,<length>) | snapList(<length>[,<length>]*)
    <tr>
      <th>Initial:
      <td>snapInterval(0px, 100%)
    <tr>
      <th>Applies to:
      <td>block-level <span class="index-def">scrollers</span>
    <tr>
      <th>Inherited:
      <td>no
    <tr>
      <th>Percentages:
      <td>Refer to the padding-box of the <span>scroller</span>
    <tr>
      <th>Media:
      <td>interactive
    <tr>
      <th>Computed&nbsp;value:
      <td>specified value
    <tr>
      <th>Animatable:
      <td>no
    <tr>
      <th>Canonical order:
      <td><abbr title="follows order of property value definition">per grammar</abbr>
  </table>

  <dl>
    <dt><dfn>''snapList(point1,point2,&hellip;)''</dfn></dt>
      <dd>Defines one or more discrete snap points for the <span>scroller</span>.
	<dt><dfn>''snapInterval(start, interval) ''</dfn></dt>
      <dd>Defines a starting snap point and an interval at which additional snap points are defined.
          The first argument is the starting snap point and the second argument is the interval to
          repeat thereafter.  </dl>
  </dl>
<h2 id="conformance">
Conformance</h2>

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

  <p>Conformance requirements are expressed with a combination of
  descriptive assertions and RFC 2119 terminology. The key words “MUST”,
  “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
  “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
  document are to be interpreted as described in RFC 2119.
  However, for readability, these words do not appear in all uppercase
  letters in this specification.

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

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

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

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

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

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

  <p>Conformance to CSS Scroll Snap Points Module 1
  is defined for three conformance classes:
  <dl>
    <dt><dfn title="style sheet!!as conformance class">style sheet</dfn>
      <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
      style sheet</a>.
    <dt><dfn>renderer</dfn></dt>
      <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
      that interprets the semantics of a style sheet and renders
      documents that use them.
    <dt><dfn id="authoring-tool">authoring tool</dfn></dt>
      <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
      that writes a style sheet.
  </dl>

  <p>A style sheet is conformant to CSS Scroll Snap Points Module 1
  if all of its statements that use syntax defined in this module are valid
  according to the generic CSS grammar and the individual grammars of each
  feature defined in this module.

  <p>A renderer is conformant to CSS Scroll Snap Points Module 1
  if, in addition to interpreting the style sheet as defined by the
  appropriate specifications, it supports all the features defined
  by CSS Scroll Snap Points Module 1 by parsing them correctly
  and rendering the document accordingly. However, the inability of a
  UA to correctly render a document due to limitations of the device
  does not make the UA non-conformant. (For example, a UA is not
  required to render color on a monochrome monitor.)

  <p>An authoring tool is conformant to CSS Scroll Snap Points Module 1
  if it writes style sheets that are syntactically correct according to the
  generic CSS grammar and the individual grammars of each feature in
  this module, and meet all other conformance requirements of style sheets
  as described in this module.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

</body>
</html>
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