Scroll-linked Animations

Unofficial Proposal Draft,

This version:
https://wicg.github.io/scroll-animations/
Issue Tracking:
Inline In Spec
GitHub Issues
Editors:
Dean Jackson <dino@apple.com>
Brian Birtles <bbirtles@mozilla.com>
Botond Ballo <botond@mozilla.com>
Mantaroh Yoshinaga <mantaroh@mozilla-japan.org>

Abstract

Defines an API and markup for creating animations that are either triggered by or tied to the scroll offset of a scroll container.

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

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 ase.

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 ase.

2.2. Scroll-triggered style changes

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

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:

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:

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:

@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:

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. Scroll-driven animations

3.1. Scroll timelines

3.1.1. The ScrollDirection enumeration

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

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

auto

If only one direction is scrollable, selects that direction. Otherwise selects the direction along the block axis.

block

Selects the direction along the block axis.

inline

Selects the direction along the inline axis.

horizontal

Selects the horizontal direction.

vertical

Selects the vertical direction.

Should the physical directions ("horizontal" and "vertical") be removed, leaving only the logical directions ("block" and "inline")?

What about a value that means, "the longest scroll direction"? That would be more reliable than "auto" for the case where layout differences could mean that, although normally you only expect the block direction to be scrollable, on some devices you end up with a small scrollable range in the inline direction too.

3.1.2. The ScrollTimeline interface

enum ScrollTimelineAutoKeyword { "auto" };

dictionary ScrollTimelineOptions {
  Element scrollSource;
  ScrollDirection orientation = "auto";
  DOMString startScrollOffset = "auto";
  DOMString endScrollOffset = "auto";
  (double or ScrollTimelineAutoKeyword) timeRange = "auto";
  FillMode fill = "none";
};

[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.

ScrollTimeline(options)

Creates a new ScrollTimeline object. The attributes of the ScrollTimeline are initialized with the respective members of the options dictionary.

scrollSource, of type Element, readonly

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

If this is not specified, the document element of the active document for the current browsing context, as determined at the time the ScrollTimeline is constructed, is used.

Note that the scrollSource's ownerDocument may change over the lifetime of the scroll timeline. This does not affect the scroll timeline; the timeline continues to be driven by the scroll progress of the scrollSource.

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 endScrollOffset.

forwards

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

backwards

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

both

When the scroll offset is less than startScrollOffset, the timeline’s current time is 0. When the scroll offset is greater than 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:

If the timeRange has the value "auto",

The effective time range is the maximum value of the target effect end of all animations directly associated with this timeline.

If any animation directly associated with the timeline has a target effect end of infinity, the effective time range is zero.

Otherwise,

The effective time range is the ScrollTimeline's timeRange.

3.1.4. The current time of a ScrollTimeline

The current time of a ScrollTimeline is calculated as follows:

  1. If scrollSource 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, return an unresolved time value if fill is none or backwards, or the effective time range otherwise.

  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.

Name: animation-timeline
Value: <single-animation-timeline>#
Initial: auto
Applies to: all elements, ::before and ::after pseudo-elements
Inherited: none
Percentages: N/A
Media: interactive
Computed value: As specified
Canonical order: per grammar
Animatable: no

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

<scroll-direction> = auto | 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;
}
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
@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.

5. Scroll-triggered (but time-driven) animations

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.

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-ANIMATIONS-1]
CSS Animations Module Level 1 URL: https://www.w3.org/TR/css3-animations/
[CSS-OVERFLOW-3]
David Baron; Florian Rivoal. CSS Overflow Module Level 3. URL: https://www.w3.org/TR/css-overflow-3/
[CSS-VALUES-3]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 3. URL: https://www.w3.org/TR/css-values-3/
[CSS-WRITING-MODES-4]
CSS Writing Modes Module Level 4 URL: https://www.w3.org/TR/css-writing-modes-4/
[CSSOM-VIEW-1]
Simon Pieters. CSSOM View Module. URL: https://www.w3.org/TR/cssom-view-1/
[DOM]
Anne van Kesteren. DOM Standard. Living Standard. URL: https://dom.spec.whatwg.org/
[HTML]
Anne van Kesteren; et al. HTML Standard. Living Standard. URL: https://html.spec.whatwg.org/multipage/
[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
[SELECTORS-4]
Selectors Level 4 URL: https://www.w3.org/TR/selectors4/
[WEB-ANIMATIONS-1]
Brian Birtles; et al. Web Animations. URL: https://www.w3.org/TR/web-animations-1/
[WebIDL]
Cameron McCormack; Boris Zbarsky; Tobie Langel. Web IDL. URL: https://www.w3.org/TR/WebIDL-1/

Informative References

[CSS3-ANIMATIONS]
Dean Jackson; et al. CSS Animations. 19 February 2013. WD. URL: https://www.w3.org/TR/css3-animations/

Property Index

Name Value Initial Applies to Inh. %ages Media Ani­mat­able Canonical order Com­puted value
animation-timeline <single-animation-timeline># auto all elements, ::before and ::after pseudo-elements none N/A interactive no per grammar As specified

IDL Index

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

enum ScrollTimelineAutoKeyword { "auto" };

dictionary ScrollTimelineOptions {
  Element scrollSource;
  ScrollDirection orientation = "auto";
  DOMString startScrollOffset = "auto";
  DOMString endScrollOffset = "auto";
  (double or ScrollTimelineAutoKeyword) timeRange = "auto";
  FillMode fill = "none";
};

[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;
};

Issues Index

Should the physical directions ("horizontal" and "vertical") be removed, leaving only the logical directions ("block" and "inline")?

What about a value that means, "the longest scroll direction"? That would be more reliable than "auto" for the case where layout differences could mean that, although normally you only expect the block direction to be scrollable, on some devices you end up with a small scrollable range in the inline direction too.