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
2.1.1. Navigation bar shrinking effect
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.
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.
2.1.2. Navigation highlight effect
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.
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.
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.
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. Scroll-driven animations
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 = "block";
orientation DOMString = "auto";
startScrollOffset DOMString = "auto"; (
endScrollOffset double or ScrollTimelineAutoKeyword )= "auto";
timeRange FillMode = "none"; }; [
fill 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.
ScrollTimeline(options)
-
Creates a new
ScrollTimeline
object using the following procedure:-
Let timeline be a new
ScrollTimeline
object. -
Let source be the result corresponding to the first matching condition from below.
- If the scrollSource value of options is non-null,
-
Let source be scrollSource
- Otherwise (scrollSource is null):
-
Let source be the
scrollingElement
of theDocument
associated with theWindow
that is the current global object.
Note: source may still be null after this step, e.g. if the
Document
has noscrollingElement
. -
Set the
scrollSource
of timeline to source. -
Assign the
orientation
,startScrollOffset
,endScrollOffset
,timeRange
, andfill
properties of timeline to the corresponding value from options.
-
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 inorientation
. - <length>
-
An absolute distance along
scrollSource
's scroll range inorientation
. - <percentage>
-
A percentage distance along
scrollSource
's scroll range inorientation
.
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 inorientation
. - <length>
-
An absolute distance along
scrollSource
's scroll range inorientation
. - <percentage>
-
A percentage distance along
scrollSource
's scroll range inorientation
.
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 bystartScrollOffset
andendScrollOffset
. 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
toendScrollOffset
, 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 toendScrollOffset
. - forwards
-
When the scroll offset is less than
startScrollOffset
, the timeline is inactive. When the scroll offset is greater than or equal to theendScrollOffset
, 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 theendScrollOffset
, 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 theendScrollOffset
, 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
'stimeRange
.
3.1.4. The current time of a ScrollTimeline
The current time of a ScrollTimeline
is calculated
as follows:
-
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. -
Otherwise, let current scroll offset be the current scroll offset of
scrollSource
in the direction specified byorientation
. -
If current scroll offset is less than
startScrollOffset
, return an unresolved time value iffill
isnone
orforwards
, or 0 otherwise. -
If current scroll offset is greater than or equal to
endScrollOffset
then:- If
endScrollOffset
is less than the maximum scroll offset ofscrollSource
inorientation
andfill
isnone
orbackwards
, -
return an unresolved time value.
- Otherwise,
-
return the effective time range.
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. - If
-
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 |
Computed value: | As specified |
Canonical order: | per grammar |
Animatable: | no |
Media: | interactive |
<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
#progress { position : fixed; top : 0 ; width : 0 ; height : 2 px ; 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(); }
@media ( prefers-reduced-motion: no-preference) { @keyframes progress{ from { width : 0 vw ; } to { width : 100 vw ; } } #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.
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
-
Does this specification deal with personally-identifiable information?
No.
-
Does this specification deal with high-value data?
No.
-
Does this specification introduce new state for an origin that persists across browsing sessions?
No.
-
Does this specification expose persistent, cross-origin state to the web?
No.
-
Does this specification expose any other data to an origin that it doesn’t currently have access to?
No.
-
Does this specification enable new script execution/loading mechanisms?
No.
-
Does this specification allow an origin access to a user’s location?
No.
-
Does this specification allow an origin access to sensors on a user’s device?
No.
-
Does this specification allow an origin access to aspects of a user’s local computing environment?
No.
-
Does this specification allow an origin access to other devices?
No.
-
Does this specification allow an origin some measure of control over a user agent’s native UI?
No.
-
Does this specification expose temporary identifiers to the web?
No.
-
Does this specification distinguish between behavior in first-party and third-party contexts?
No.
-
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.
-
Does this specification persist data to a user’s local device?
No.
-
Does this specification have a "Security Considerations" and "Privacy Considerations" section?
Yes.
-
Does this specification allow downgrading default security characteristics?
No.