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, 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.
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.
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:
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:
The suppression window begins at the start of the current iteration of the HTML Processing Model event loop,
or at the end of the most recently completed suppression window,
whichever is more recent.
The suppression window ends at the end of the current iteration of the HTML Processing Model event loop,
or immediately before the next operation whose result or side effects
would differ as a result of a change in the scroll position
(for example, an invocation of getBoundingClientRect()),
whichever comes sooner.
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:
Any change to the computed value of any of the following properties,
on any element in the path from the anchor node to the scrollable element (or document),
inclusive of both:
Any change to the computed value of the position property
on any element within the scrollable element (or document),
such that the element becomes or stops being absolutely positioned.
Note that this trigger applies regardless of whether the modified element is
on the path from the anchor node to the scrollable element.
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.
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.
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.
as it only adjusts how we compute scroll positions,
introduces no new privacy or security considerations.
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",
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 to this specification
is defined for three conformance classes:
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.
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
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