[scroll-animations-1] Add self keyword

[bramus]

Fixes #8227.

This PR replaces #8390 which was created from the wrong branch.

[fantasai]

You need to define what happens when the element's own box isn't a [=scroll container=].

Also you need to specify [=principal box=]. Some elements have multiple boxes.

[bramus]

Thanks for the feedback @fantasai; greatly appreciate it. Did some changes which should address your remarks. Happy to take more pointers, as this is all fairly new to me.

[fantasai]

Looks pretty good! The one other thing to fix is the line breaks.
Read https://rhodesmill.org/brandon/2012/one-sentence-per-line/ ; then take a look at the rest of the document to get a sense of how we're formatting the source.

[bramus]

Interesting link. Makes sense, instead of cutting off mid-sentence when the line length limit is almost reached. Think I got it right now.

[flackr]

You need to define what happens when the element's own box isn't a [=scroll container=].

I would have thought these cases are covered by the text in the ScrollTimeline interface:

If the source of a ScrollTimeline is an element whose principal box does not exist or is not a scroll container, or if there is no scrollable overflow, then the ScrollTimeline is inactive.

Do we define this for scroll-timeline-name as well since that can target a non scroll container?

[bramus]

@fantasai I’ve committed the suggested change.