[csswg-drafts] Pull Request: [scroll-animations-1] Add Element-based offset (#4337)

majido has just labeled a pull request from majido for https://github.com/w3c/csswg-drafts as "scroll-animations":

== [scroll-animations-1] Add Element-based offset (#4337) ==
Add basic definition for Element-based offsets

Major changes:
* Introduce concept of "scroll timeline offset" that can be container-based (existing concept) and element-based (new concept).
* Add IDL for the new offset type and use it.
* Define the process for each offset type to be resolved into an effective scroll offset.
* Update current time calculation to resolve offsets and use the effective values.

Minor changes:
 * Rewrap lines to fit in 80 chars
 * Trim end-of-line whitespace
 * Clarify some definitions

TODO (as follow ups):
 - Define threshold for element-based offset.
 - Add a diagram to visualize the element-based computation.
 - Add css syntax for element-based offsets.


See https://github.com/w3c/csswg-drafts/pull/5124