CSS Scroll Snap Points Module Level 1

Abstract

Status of this document

Table of Contents

1. Introduction

This section is not normative.

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.

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.

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

1.1. Module interactions

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.

1.2. Values

This specification follows the CSS property definition conventions 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 <length> value type as used in this specification.

2. Motivating Examples

In this example, a series of images arranged in a scroll container are used to build a photo gallery. The scroll container 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 scroll container’s 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.

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;
}

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

This example also builds a photo gallery as in example 1. However, in this example the scroll container 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 scroll container’s visual viewport.

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;
}

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

This example builds a paginated document that aligns each page near to (but not exactly on) the edge of the scroll container. 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.

.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;
}

<div class="docScroller">
    <div class="page">Page 1</div>
    <div class="page">Page 2</div>
    <div class="page">Page 3</div>
    <div class="page">Page 4</div>
</div>

3. Definitions

scroll container

An element which provides a scrolling user interface as described in [CSS21], particularly in the section on overflow.

4. Scroll Snap Types: the scroll-snap-type property

The scroll-snap-type property is used to define how strictly snap points are enforced on the scroll container, 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.

none

The visual viewport of this scroll container must ignore snap points, if any, when scrolled.

mandatory

The visual viewport of this scroll container 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.

proximity

The visual viewport of this scroll container 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.

5. Scroll Snap Points: the scroll-snap-points-x and scroll-snap-points-y properties

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 scroll container they are applied to.

none

No snap points are defined by this scroll container. Contained elements may still define snap points on this scroll container’s behalf.

repeat(<length>)

Defines an interval at which snap points are defined, starting from the container’s relevant start edge. Only positive lengths are allowed.

6. Scroll Snap Destination: the scroll-snap-destination property

The scroll-snap-destination property is used to define the x and y coordinate within the scroll container’s visual viewport which element snap points will align with.

<position>

Specifies the offset of the snap destination from the start edge of the scroll container’s visual viewport. The first value gives the x coordinate of the snap destination, the second value its y coordinate.

7. Scroll Snap Coordinate: the scroll-snap-coordinate property

The scroll-snap-coordinate property is used to define the x and y coordinate within the element which will align with the nearest ancestor scroll container’s 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).

How does this work with fragmentation?

Consider alternative naming besides "coordinate". Consider naming conventions like in Grid Layout for grouping properties on the container vs. items.

none

Specifies that this element does not contribute a snap point.

<position>#

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.

How can an alternative box be specified?

Acknowledgments

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

Conformance

Document conventions

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 class="example", like this:

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

Note, this is an informative note.

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.

Conformance classes

Conformance to this specification is defined for three conformance classes:

style sheet

A CSS style sheet.

renderer

A UA that interprets the semantics of a style sheet and renders documents that use them.

authoring tool

A UA that writes a style sheet.

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.

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.)

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.

Partial implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not 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.

Experimental implementations

To avoid clashes with future CSS features, the CSS2.1 specification reserves a prefixed syntax 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.

Non-experimental implementations

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 http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.

References

Normative References

[CSS21]

Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-CSS2-20110607

[rfc2119]

S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: http://www.ietf.org/rfc/rfc2119.txt

Informative References

[CSS3VAL]

Håkon Wium Lie; Tab Atkins; Elika J. Etemad. CSS Values and Units Module Level 3. 30 July 2013. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2013/CR-css3-values-20130730/

Index

• mandatory, 4
• none
  • value for scroll-snap-type, 4
  • value for snap-points-x,, snap-points-y, 5
  • value for scroll-snap-coordinate, 7
• <position>, 6
• <position>#, 7
• proximity, 4
• repeat(<length>), 5
• scroll container, 3
• scroll-snap-coordinate, 7
• scroll-snap-destination, 6
• scroll-snap-points-x, 5
• scroll-snap-points-y, 5
• scroll-snap-type, 4

Property Index

Issues Index