Overview.src.html

<h1>CSS Scroll Snap Points Module Level 1</h1>
<pre class='metadata'>
Group: csswg
Shortname: css-snappoints
Level: 1
Status: ED
ED: http://dev.w3.org/csswg/css-snappoints/
Editor: Matt Rakow, Microsoft
Editor: Jacob Rossi, Microsoft
Abstract: This module contains features to control panning and scrolling behavior with “snap points”.
!Issue Tracking: <a href="http://wiki.csswg.org/spec/css-snappoints">http://wiki.csswg.org/spec/css-snappoints</a>
</pre>

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

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

	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.

	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.

	To this end, we introduce scroll snap points which enforce the scroll offsets that a scroll container may end at after a scrolling operation has completed.

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

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

	None of the properties in this module apply to the ''::first-line'' and ''::first-letter'' pseudo-elements.

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

	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></var> value type as used in this specification.
	
	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">
Motivating Examples</h2>

	<div class=example>

		In this example, a series of images arranged in a <span>scroll container</span> are used to build a photo gallery. 
		The <span>scroll container</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>scroll container</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.

		<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>

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

		<figure>
			<img src="example1.png" alt="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 containing block's width.">
	
			<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 containing block's width.
			</figcaption>
		</figure>
	</div>  

<h2 id="definitions">
Definitions</h2>

	<dl>
		<dt><dfn export>scroll container</dfn>
		<dd>
			An element which provides a scrolling user interface as described in [[!CSS21]], particularly in the section on overflow. 
	</dl>

<h2 id="scroll-snap-type">
Scroll Snap Types: the 'scroll-snap-type' property</h2>

	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 scroll container it is applied to.

	<pre class='propdef'>
	Name: scroll-snap-type
	Value: none | mandatory | proximity
	Initial: none
	Applies to: <a>scroll containers</a>
	Inherited: no
	Percentages: n/a
	Media: interactive
	Computed value: specified value
	Animatable: no
	</pre>
	
	<dl dfn-type="value" dfn-for="scroll-snap-type">
		<dt><dfn>none</dfn>
		<dd>
			This <a>scroll container</a> must ignore snap points, if any, when scrolled.

		<dt><dfn>mandatory</dfn>
		<dd>
			This <a>scroll container</a> must come to rest on a snap point at the termination of a scroll, if possible.

		<dt><dfn>proximity</dfn>
		<dd>
			This <a>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.
	</dl>

<h2 id="scroll-snap-points">
Scroll Snap Points: the 'scroll-snap-points-x' and 'scroll-snap-points-y properties</h2>

	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 <a>scroll container</a> they are applied to.

	<pre class="propdef">
	Name: scroll-snap-points-x, scroll-snap-points-y
	Value: snapInterval(<<length>>, <<length>>) | snapList(<<length>>)
	<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">scroll containers</span>
		<tr>
			<th>Inherited:
			<td>no
		<tr>
			<th>Percentages:
			<td>Refer to the padding-box of the <span>scroll container</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>
			<dd>Defines one or more discrete snap points for the <span>scroll container</span>. 
	<dt><dfn>''snapInterval(start, interval) ''</dfn>
			<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>

	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.
	
	All of the text of this specification is normative except sections
	explicitly marked as non-normative, examples, and notes. [[!RFC2119]]
	
	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">
		This is an example of an informative example.
	</div>
	
	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.

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

	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>
			<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>
			<dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
			that writes a style sheet.
	</dl>
	
	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.
	
	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.)
	
	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>

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

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

	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.
	
	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.
	
	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>

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

<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>
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