1. Introduction

This specification defines mechanisms for driving the progress of an animation based
on the scroll progress of a scroll container.

1.1. Relationship to other specifications

Web Animations [WEB-ANIMATIONS-1] defines an abstract conceptual model for
animations on the Web platform, with elements of the model including animations and their timelines,
and associated programming interfaces.

This specification extends this model by defining a new type of animation timeline:
a scroll timeline.

This specification defines both programming interfaces for interacting with these
concepts, as well as CSS markup which applies these concepts to CSS Animations [CSS3-ANIMATIONS].

The behavior of the CSS markup is described in terms of the programming interfaces.
User agents that do not support script may still implement the CSS markup
provided it behaves as if the underlying programming interfaces were in place.

1.2. Relationship to asynchronous scrolling

Some user agents support scrolling that’s asynchronous with respect to layout
or script. This specification is intended to be compatible with such an
architecture.

Specifically, this specification allows expressing scroll-linked effects in a
way that does not require script to run each time the effect is sampled. User
agents that support asynchronous scrolling are allowed (but not required) to
sample such effects asynchronously as well.

2. Use cases

This section is non-normative

Note: Based on this curated
list of use cases
.

2.1. Scroll-triggered animations

It is common to trigger an animation to run when the scroll position
reaches a certain point. For example, a navigation bar may shrink once
the user begins to scroll a page.

Use case: Shrinking navigation bar
Shrinking navigation bar
The left figure shows the navigation bar before scrolling with a large menu bar.
The right figure shows the shrunken navigation bar after scrolling.

The proposal does not yet define CSS markup or programming interfaces to express this use case.

Similarly, it is common to trigger an animation at certain fixed points
in a element’s scroll range. For example, a navigation bar that changes
highlight based on the reader’s position within the document.

Use case: A navigation highlight effect.
A navigation highlight effect
On the left, the “Abstract” section is scrolled into view and hence the abstract menu item is highlighted.
After scrolling down to the “Background” section (right), the background menu item fades in while the abstract menu item fades out.

The proposal does not yet define CSS markup or programming interfaces to express this use case.

2.2. Scroll-triggered style changes

The proposal does not yet define CSS markup or programming interfaces to express this use case.

2.3. Scroll-linked animations

2.3.1. Scrollable picture-story show

Another pattern is an animation that tells a story where the user
controls the progress of the animation by scrolling or some other
gesture. This may be because the animation contains a lot of textual
information which the user may wish to peruse more slowly, it may be for
accessibility considerations to accommodate users who are uncomfortable
with rapid animation, or it may simply be to allow the user to easily
return to previous parts of the story such as a story that introduces
a product where the user wishes to review previous information.

The following (simplified) example shows two balls colliding. The
animation is controlled by scroll position allowing the user to easily
rewind and replay the interaction.

Use case: The picture-story show.
A scrollable movie.
The left figure shows the initial position of the balls
The right figure shows them after they have collided.

Using the CSS markup:

@media (prefers-reduced-motion: no-preference) { div.circle { animation-duration: 1s; animation-timing-function: linear; animation-timeline: scroll(element(#container), vertical, "200px", "300px"); } #left-circle { animation-name: left-circle; } #right-circle { animation-name: right-circle; } #union-circle { animation-name: union-circle; animation-timeline: scroll(element(#container), vertical, "250px", "300px"); } @keyframes left-circle { to { transform: translate(300px) } } @keyframes right-circle { to { transform: translate(350px) } } @keyframes union-circle { to { opacity: 1 } }
}

Using the programming interface, we might write this as:

if (window.matchMedia('(prefers-reduced-motion: no-preference)').matches) { var circleTimeline = new ScrollTimeline({ scrollSource: scrollableElement, scrollOffset: '200px', endScrollOffset: '300px' }); var left = leftCircle.animate({ transform: 'translate(300px)' }, 1000); left.timeline = circleTimeline; var right = leftCircle.animate({ transform: 'translate(350px)' }, 1000); right.timeline = circleTimeline; var union = unionCircle.animate({ opacity: 1 }, 1000); union.timeline = new ScrollTimeline({ scrollSource: scrollableElement, startScrollOffset: '250px', endScrollOffset: '300px' });
}

2.3.2. The content progress bar

Another common example of an animation that tracks scroll position is a
progress bar that is used to indicate the reader’s position in a long
article.

Use case: Scroll based styling
Content progress bar.
The left figure shows the initial state before scrolling.
The right figure shows the progress bar is half-filled in since the user has scrolled half way through the article.

Typically, the scroll bar provides this visual indication but
applications may wish to hide the scroll bar for aesthetic or useability
reasons.

Using the animation-timeline property, this example could be written
as follows:

@media (prefers-reduced-motion: no-preference) { @keyframes progress { to { width: 100%; } } #progress { width: 0px; height: 30px; background: red; animation: progress 1s linear; animation-timeline: scroll(element(#body)); }
}

If we use this API for this case, the example code will be as follow:

if (window.matchMedia('(prefers-reduced-motion: no-preference)').matches) { var animation = div.animate({ width: '100%' }, 1000); animation.timeline = new ScrollTimeline( { startScrollOffset: '0px' } );
}

2.4. Combination scroll and time-base animations

2.4.1. Photo viewer

We are currently reworking this use case

3.1. Scroll timelines

3.1.1. The ScrollDirection enumeration

enum ScrollDirection { "block", "inline", "horizontal", "vertical"
};

The ScrollDirection enumeration specifies a direction of scroll of a
scrollable element.

block

Selects the direction along the block axis, conforming to writing mode
and directionality.

inline

Selects the direction along the inline axis, confirming to writing mode
and directionality.

horizontal

Selects the physical horizontal direction (ignoring writing mode and
directionality).

vertical

Selects the physical vertical direction (ignoring writing mode and
directionality).

Note: Having both logical (block/inline) and physical (vertical/horizontal)
directions allows web developers to animate both logical (e.g.
margin-inline-start) and physical (e.g. transform) properties with good
behavior under different directionalities and writing modes.

3.1.2. The ScrollTimeline interface

enum ScrollTimelineAutoKeyword { "auto" }; dictionary ScrollTimelineOptions { Element? scrollSource = null; ScrollDirection orientation = "block"; DOMString startScrollOffset = "auto"; DOMString endScrollOffset = "auto"; (double or ScrollTimelineAutoKeyword) timeRange = "auto"; FillMode fill = "none";
}; [Exposed=Window, Constructor(optional ScrollTimelineOptions options)]
interface ScrollTimeline : AnimationTimeline { readonly attribute Element scrollSource; readonly attribute ScrollDirection orientation; readonly attribute DOMString startScrollOffset; readonly attribute DOMString endScrollOffset; readonly attribute (double or ScrollTimelineAutoKeyword) timeRange; readonly attribute FillMode fill;
};

A scroll timeline is an AnimationTimeline whose time values are determined
not by wall-clock time, but by the progress of scrolling in a scroll container.

scrollSource, of type Element, readonly

The scrollable element whose scrolling triggers the activation and drives the
progress of the timeline.

orientation, of type ScrollDirection, readonly

Determines the direction of scrolling which triggers the activation and drives
the progress of the trigger.

startScrollOffset, of type DOMString, readonly

A scroll offset, in the direction specified by orientation, which constitutes
the beginning of the range in which the timeline is active.

Recognized values are defined by the following grammar:

auto | <length> | <percentage>

The meaning of each value is as follows:

auto

The beginning of scrollSource‘s scroll range in orientation.

<length>

An absolute distance along scrollSource‘s scroll range in orientation.

<percentage>

A percentage distance along scrollSource‘s scroll range in orientation.

endScrollOffset, of type DOMString, readonly

A scroll offset, in the direction specified by orientation, which constitutes
the end of the range in which the trigger is activated.

Recognized values are defined by the following grammar:

auto | <length> | <percentage>

The meaning of each value is as follows:

auto

The end of scrollSource‘s scroll range in orientation.

<length>

An absolute distance along scrollSource‘s scroll range in orientation.

<percentage>

A percentage distance along scrollSource‘s scroll range in orientation.

timeRange, of type (double or ScrollTimelineAutoKeyword), readonly

A time duration that allows mapping between a distance scrolled, and
quantities specified in time units, such as an animation’s duration and start delay.

Conceptually, timeRange represents the number of milliseconds to map to the
scroll range defined by startScrollOffset and endScrollOffset. As a result, this value does not have a correspondence to wall-clock time.

This value is used to compute the timeline’s effective time range, and
the mapping is then defined by mapping the scroll distance from startScrollOffset to endScrollOffset, to the effective time range.

fill, of type FillMode, readonly

Determines whether the timeline is active even when the scroll offset is outside
the range defined by [startScrollOffset, endScrollOffset].

Possible values are:

none

The timeline is inactive when the scroll offset is less than startScrollOffset or greater than or equal to endScrollOffset.

forwards

When the scroll offset is less than startScrollOffset, the
timeline is inactive.
When the scroll offset is greater than or equal to the endScrollOffset, the timeline’s current time is its effective time range.

backwards

When the scroll offset is less than startScrollOffset, the
timeline’s current time is 0.
When the scroll offset is greater than or equal to the endScrollOffset, the timeline is inactive.

both

When the scroll offset is less than startScrollOffset, the
timeline’s current time is 0.
When the scroll offset is greater than or equal to the endScrollOffset, the timeline’s current time is its effective time range.

auto

Behaves the same as both.

3.1.3. The effective time range of a ScrollTimeline

The effective time range of a ScrollTimeline is calculated as follows:

3.1.4. The current time of a ScrollTimeline

The current time of a ScrollTimeline is calculated
as follows:

  1. If scrollSource is null, does not currently have a CSS layout box, or
    if its layout box is not a scroll container, return an unresolved time value.

  2. Otherwise, let current scroll offset be the current scroll offset of scrollSource in the direction specified by orientation.

  3. If current scroll offset is less than startScrollOffset, return an unresolved
    time value if fill is none or forwards, or 0 otherwise.

  4. If current scroll offset is greater than or equal to endScrollOffset then:

    Note: Checking for endScrollOffset being the maximum scroll offset ensures that the common
    case of a ‘whole scroller’ ScrollTimeline does not become inactive when you scroll to the end.

  5. Return the result of evaluating the following expression:

    (current scroll offset - startScrollOffset) / (endScrollOffset - startScrollOffset) × effective time range

3.2. The animation-timeline property

A ScrollTimeline may be applied to a CSS Animation [CSS3-ANIMATIONS] using
the animation-timeline property.

<single-animation-timeline> = auto | scroll([element(<id-selector>)[, <scroll-direction>[, <scroll-offset>[, <scroll-offset>[, <time>[, <single-animation-fill-mode>]]]]]])

<scroll-direction> = auto | block | inline | horizontal | vertical

<scroll-offset> = <length> | <percentage> | auto

The animation-timeline property is similar to properties like animation-duration and animation-timing-function in that it can have one or more values, each one imparting additional behavior to a corresponding animation on the element, with the timelines matched up with animations as described here.

Each value has type <single-animation-timeline>, whose possible values have the
following effects:

auto

The animation’s timeline is a DocumentTimeline, more specifically the default document timeline.

scroll([element(<id-selector>)[, <scroll-direction>[, <scroll-offset>[, <scroll-offset>[, <time>[, <single-animation-fill-mode>]]]]]])

The animation’s timeline is a ScrollTimeline.

The timeline’s scrollSource is the scroll container identified
by the <id-selector>, defaulting to the element’s nearest scrollable ancestor.

The <scroll-direction>, if provided, determines the timeline’s orientation.

The first <scroll-offset>, if provided, determines the timeline’s startScrollOffset.

The second <scroll-offset>, if provided, determines the timeline’s endScrollOffset.

The <time> value, if specified, determines the timeline’s timeRange.

The <single-animation-fill-mode> value, if specified, determines the timeline’s fill.

3.3. Examples

Draw a reading progress bar along the top of the page as the user scrolls

#progress { position: fixed; top: 0; width: 0; height: 2px; background-color: red;
}
if (window.matchMedia('(prefers-reduced-motion: no-preference)').matches) { let progress = document.getElementById("progress"); let effect = new KeyframeEffect( progress, [ { width: "0vw" }, { width: "100vw" } ], { duration: 1000, easing: "linear" }); let timeline = new ScrollTimeline({ trigger: new ScrollTrigger({ scrollSource: document.documentElement, orientation: "vertical", kind: "range" }); }); let animation = new Animation(effect, timeline); animation.play();
}

The same thing with CSS, using animation-timeline

@media (prefers-reduced-motion: no-preference) { @keyframes progress { from { width: 0vw; } to { width: 100vw; } } #progress { position: fixed; top: 0; width: 0; height: 2px; background-color: red; animation-name: progress; animation-duration: 1s; animation-timing-function: linear; /* Assume the HTML element has id 'root' */ animation-timeline: scroll(element(#root), vertical); }
}

4. Avoiding cycles with layout

The ability for scrolling to drive the progress of an animation, gives rise to
the possibility of layout cycles, where a change to a scroll offset
causes an animation’s effect to update, which in turn causes a new change to the
scroll offset.

To avoid such cycles, animations with a ScrollTimeline are sampled once
per frame, after scrolling in response to input events has taken place, but
before requestAnimationFrame() callbacks are run. If the sampling of such an
animation causes a change to a scroll offset, the animation will not be
re-sampled to reflect the new offset until the next frame.

The implication of this is that in some situations, in a given frame, the
rendered scroll offset of a scroll container may not be consistent with the state of an animation driven by scrolling that scroll container. However, this will
only occur in situations where the animation’s effect changes the scroll offset
of that same scroll container (in other words, in situations where the animation’s
author is asking for trouble). In normal situations, including – importantly – when scrolling happens in response to input events, the rendered scroll offset
and the state of scroll-driven animations will be consistent in each frame.

User agents that composite frames asynchronously with respect to layout and/or
script may, at their discretion, sample scroll-driven animations once per composited frame, rather than (or in addition to) once per full layout cycle. Again, if sampling such an animation causes a change to a scroll offset, the animation will not be re-sampled to reflect the new offset until the next frame.

Nothing in this section is intended to require that scrolling block on layout
or script. If a user agent normally composites frames where scrolling has
occurred but the consequences of scrolling have not been fully propagated in
layout or script (for example, scroll event listeners have not yet run), the user agent may likewise choose not to sample scroll-driven animations
for that composited frame. In such cases, the rendered scroll offset and the
state of a scroll-driven animation may be inconsistent in the composited frame.

An earlier draft of this proposal also provided for animations whose progress
was driven by time (as with existing animations), but whose activation was triggered by scrolling past a certain scroll offset or into a given scroll range.

The main objective was to allow triggering the animation from the compositor
thread. (The objective of scroll-linked animations is to make sure that the animation is in sync with the scroll position on each composited frame.
If the triggering doesn’t happen on the compositor thread, then it’s possible
that for a few frames the visual scroll position is such that the animation
should have started, but it has not in fact started yet because the main thread,
which is doing the triggering, is lagging behind.)

However, we found that in the vast majority of cases where a web author would
want to do this, they would want to do it for a CSS transition (as opposed to
a CSS animation). Unfortunately, it’s not possible to trigger CSS transitions from the compositor thread (because triggering a transition requires style resolution, which cannot be performed on the compositor thread). Given the extent to which triggering complicated the API, we decided it wasn’t worth it if you can’t use it for transitions, so this feature was removed.

The design space for triggering animations is still open. We welcome input
on this subject.

Appendix A. Considerations for Security and Privacy

This appendix is informative.

There are no known security or privacy impacts of this feature.

The W3C TAG is developing a Self-Review Questionnaire: Security and Privacy for editors of specifications to informatively answer.

Per the Questions to Consider

  1. Does this specification deal with personally-identifiable information?

    No.

  2. Does this specification deal with high-value data?

    No.

  3. Does this specification introduce new state for an origin that persists across browsing sessions?

    No.

  4. Does this specification expose persistent, cross-origin state to the web?

    No.

  5. Does this specification expose any other data to an origin that it doesn’t currently have access to?

    No.

  6. Does this specification enable new script execution/loading mechanisms?

    No.

  7. Does this specification allow an origin access to a user’s location?

    No.

  8. Does this specification allow an origin access to sensors on a user’s device?

    No.

  9. Does this specification allow an origin access to aspects of a user’s local computing environment?

    No.

  10. Does this specification allow an origin access to other devices?

    No.

  11. Does this specification allow an origin some measure of control over a user agent’s native UI?

    No.

  12. Does this specification expose temporary identifiers to the web?

    No.

  13. Does this specification distinguish between behavior in first-party and third-party contexts?

    No.

  14. How should this specification work in the context of a user agent’s “incognito” mode?

    No differently. The website should not be able to determine that the user is
    in an “incognito” mode using scroll-linked animations.

  15. Does this specification persist data to a user’s local device?

    No.

  16. Does this specification have a “Security Considerations” and “Privacy Considerations” section?

    Yes.

  17. Does this specification allow downgrading default security characteristics?

    No.

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.

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.

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.

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.

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