CSS Scroll Anchoring Module Level 1

Editor’s Draft,

Specification Metadata
This version:
https://drafts.csswg.org/css-scroll-anchoring
Editors:
Steve Kobes (Google)
Tab Atkins-Bittner (Google)
Issue Tracking:
GitHub Issues

Abstract

Changes in DOM elements above the visible region of a scrolling box can result in the page moving while the user is in the middle of consuming the content.

This spec proposes a mechanism to mitigate this jarring user experience by keeping track of the position of an anchor node and adjusting the scroll offset accordingly.

This spec also proposes an API for web developers to opt-out of this behavior.

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc.

Status of this document

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.

GitHub Issues are preferred for discussion of this specification. When filing an issue, please put the text “css-scroll-anchoring” in the title, preferably like this: “[css-scroll-anchoring] …summary of comment…”. All issues and comments are archived, and there is also a historical archive.

This document was produced by the CSS Working Group (part of the Style Activity).

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures 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 Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 1 March 2017 W3C Process Document.

1. Introduction

Today, users of the web are often distracted by content moving around due to changes that occur outside the viewport. Examples include script inserting an iframe containing an ad, or non-sized images loading on a slow network.

Historically the browser’s default behavior has been to preserve the absolute scroll position when such changes occur. This means that to avoid shifting content, the webpage can attempt to reserve space on the page for anything that will load later. In practice, few websites do this consistently.

Scroll anchoring aims to minimize surprising content shifts. It does this by adjusting the scroll position to compensate for the changes outside the viewport.

The explainer document gives an informal overview of scroll anchoring.

2. Description

Scroll anchoring works by selecting a DOM node (the anchor node) whose movement is used to determine adjustments to the scroll position.

2.1. Anchor Node Selection

Each scrolling box aims to select an anchor node that is deep in the DOM and close to the block start edge of its optimal viewing region.

Note: If the user agent does not support the scroll-padding property, the optimal viewing region of the scrolling box is equivalent to its content area.

The anchor node is either a non-anonymous block box or a text node. The anchor node is always a descendant of the scrolling box. In some cases, a scrolling box may not select any anchor node.

The anchor node selection algorithm for a scrolling box S is as follows:
  1. If S is associated with an element whose computed value of the overflow-anchor property is none, then do not select an anchor node for S.

  2. Otherwise, for each DOM child N of the element or document associated with S, perform the candidate examination algorithm for N in S, and terminate if it selects an anchor node.

The candidate examination algorithm for a candidate DOM node N in a scrolling box S is as follows:
  1. If N is an excluded subtree, or if N is fully clipped in S, then do nothing (N and its descendants are skipped).

  2. If N is fully visible in S, select N as the anchor node.

  3. If N is partially visible:

    1. For each DOM child C of N, perform the candidate examination algorithm for C in S, and terminate if it selects an anchor node.

    2. For each absolutely-positioned element A whose containing block is N, but whose DOM parent is not N, perform the candidate examination algorithm for A in S, and terminate if it selects an anchor node.

    3. Select N as the anchor node. (If this step is reached, no suitable anchor node was found among N’s descendants.)

    Note: Deeper nodes are preferred to minimize the possibility of content changing inside the anchor node but outside the viewport, which would cause visible content to shift without triggering any scroll anchoring adjustment.

Conceptually, a new anchor node is computed for every scrolling box whenever the scroll position of any scrolling box changes. (As a performance optimization, the implementation may wait until the anchor node is needed before computing it.)

A DOM node N is an excluded subtree if it is an element and any of the following conditions holds:
A DOM node N is fully visible in a scrolling box S if N’s scroll anchoring bounding rect is entirely within the optimal viewing region of S.
A DOM node N is fully clipped in a scrolling box S if N’s scroll anchoring bounding rect is entirely outside the optimal viewing region of S.
A DOM node N is partially visible in a scrolling box S if N is neither fully visible in S nor fully clipped in S.
The scroll anchoring bounding rect of a DOM node N is N’s scrollable overflow rectangle if N is a block box, or the bounding rect of its line boxes if N is a text node.

2.2. Scroll Adjustment

If an anchor node was selected, then when the anchor node moves, the browser computes the previous offset y0, and the current offset y1, of the block start edge of the anchor node’s scroll anchoring bounding rect, relative to the block start edge of the scrolling content in the block flow direction of the scroller.

It then queues an adjustment to the scroll position of y1 - y0, in the block flow direction, to be performed at the end of the suppression window.

The scroll adjustment is a type of [[cssom-view-1#scrolling-events#scrolling]] as defined by [CSSOM-VIEW], and generates scroll events in the manner described there.

2.2.1. Suppression Window

Every movement of an anchor node occurs within a window of time called the suppression window, defined as follows:

Note: The suppression window boundaries should be incorporated into the HTML standard once the scroll anchoring API is stabilized.

More than one anchor node movement may occur within the same suppression window.

At the end of a suppression window, the user agent performs all scroll adjustments that were queued during the window and not suppressed by any suppression trigger during the window.

2.2.2. Suppression Triggers

A suppression trigger is an operation that suppresses the scroll anchoring adjustment for an anchor node movement, if it occurs within the suppression window for that movement. These triggers are:

Note: Suppression triggers exist for compatibility with existing web content that has negative interactions with scroll anchoring due to shifting content in scroll event handlers.

3. Exclusion API

Scroll anchoring aims to be the default mode of behavior when launched, so that users benefit from it even on legacy content. overflow-anchor can disable scroll anchoring in part or all of a webpage (opt out), or exclude portions of the DOM from the anchor node selection algorithm.

Name: overflow-anchor
Value: auto | none
Initial: auto
Applies to: all elements
Inherited: no
Percentages: n/a
Media: visual
Computed value: as specified
Canonical order: per grammar
Animation type: discrete

Values are defined as follows:

auto

Declares that the element is potentially eligible to participate in the anchor node selection algorithm for any scrolling box created by the element or an ancestor.

none

Declares that the element and its descendants (that aren’t nested inside of another scrolling element) are not eligible to participate in the anchor node selection algorithm for any scrolling box created by the element or an ancestor.

Note: It is not possible to turn scroll anchoring "back on" for descendants of a overflow-anchor: none element. However, descendant scroll containers automatically "turn it back on" (for their own scrolling box) unless they explicitly have overflow-anchor: none set on them as well.

Note: The overflow-anchor property was also proposed (with different values) for CSS Sticky Scrollbars, which has now been superseded.

4. Privacy and Security Considerations

This specification, as it only adjusts how we compute scroll positions, introduces no new privacy or security considerations.

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:

This is an example of an informative example.

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.

Requirements for Responsible Implementation of CSS

The following sections define several conformance requirements for implementing CSS responsibly, in a way that promotes interoperability in the present and future.

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 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.

Implementations of Unstable and Proprietary Features

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

Implementations of CR-level Features

Once a specification reaches the Candidate Recommendation stage, implementers should release an unprefixed 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.

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.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS-DISPLAY-3]
Elika Etemad. CSS Display Module Level 3. 20 July 2017. WD. URL: https://www.w3.org/TR/css-display-3/
[CSS-OVERFLOW-3]
David Baron; Florian Rivoal. CSS Overflow Module Level 3. 31 May 2016. WD. URL: https://www.w3.org/TR/css-overflow-3/
[CSS-POSITION-3]
Rossen Atanassov; Arron Eicholz. CSS Positioned Layout Module Level 3. 17 May 2016. WD. URL: https://www.w3.org/TR/css-position-3/
[CSS-SCROLL-SNAP-1]
Matt Rakow; et al. CSS Scroll Snap Module Level 1. 24 August 2017. CR. URL: https://www.w3.org/TR/css-scroll-snap-1/
[CSS-TRANSFORMS-1]
Simon Fraser; et al. CSS Transforms Module Level 1. 26 November 2013. WD. URL: https://www.w3.org/TR/css-transforms-1/
[CSS22]
Bert Bos. Cascading Style Sheets Level 2 Revision 2 (CSS 2.2) Specification. 12 April 2016. WD. URL: https://www.w3.org/TR/CSS22/
[CSSOM-VIEW]
Simon Pieters. CSSOM View Module. 17 March 2016. WD. URL: https://www.w3.org/TR/cssom-view-1/
[DOM]
Anne van Kesteren. DOM Standard. Living Standard. URL: https://dom.spec.whatwg.org/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119

Informative References

[CSS3-EXCLUSIONS]
Rossen Atanassov; Vincent Hardy; Alan Stearns. CSS Exclusions Module Level 1. 15 January 2015. WD. URL: https://www.w3.org/TR/css3-exclusions/

Property Index

Name Value Initial Applies to Inh. %ages Media Anim­ation type Canonical order Com­puted value
overflow-anchor auto | none auto all elements no n/a visual discrete per grammar as specified