[scroll-animations-1] Use pseudo-element as the source of ScrollTimeline or subject of ViewTimeline

[BorisChiou]

Per the spec [scroll-animations-1] definition, ScrollTimeline interface has an attribute, source, which is the scroll container of this ScrollTimeline:

[Exposed=Window]
interface ScrollTimeline : AnimationTimeline {
  constructor(optional ScrollTimelineOptions options = {});
  readonly attribute Element? source;
  readonly attribute ScrollAxis axis;
};

However, what happened if the scroll container is a pseudo-element? Should we add a CSSOMString to represent the pseudo-element, just like KeyframeEffect? Or restrict it to non pseudo-element?

Same question for attribute subject in ViewTimeline. We only have an Element attribute, and I think it is possible to let a pseudo-element be a subject, right? Or do we have to restrict it for only non pseudo-element?

[Exposed=Window]
interface ViewTimeline : ScrollTimeline {
  constructor(optional ViewTimelineOptions options = {});
  readonly attribute Element subject;
  ...
};

[BorisChiou]

cc @dshin-moz @emilio @flackr @jwatt

Also note that I saw a update in Chromium which let the source be the originating element if the scroll container is the pseudo-element.

[flackr]

We should follow the same convention as with web-animations. You can animate pseudo-elements but when you look at the target element you get the originating element. For scroll timelines this means until we add a pseudo attribute it will not be possible to tell whether the scroll timeline is on the pseudo element or the originating element from JS.

[BorisChiou]

Thanks. This makes sense to me. For now in Gecko I am also planning to use originating element as source or subject until we introduce an extra attribute to represent the pseudo element type.