[web-animations] How/when do the "rangeStart/rangeEnd" attributes the Animation interface get evaluated

[weinig]

How/when do the <length-percentage> values that can be passed to the rangeStart/rangeEnd attributes on the Animation interface (or via KeyframeAnimationOption) when assigning a DOMString, get resolved.

Specifically, the KeyframeAnimationOption section for rangeEnd says "A DOMString value is parsed as an animation-range-end value to yield a TimelineRangeOffset". animation-range-end is defined as [ normal | <length-percentage> | <timeline-range-name> <length-percentage>? ]#, so, <length-percentage> values will have to resolved.

Given all that, how/when should a string like 1em be resolved?

[ydaniv]

I suppose you have to have an active timeline in order to resolve percentages.
So I assume whenever both of the following two apply:

1. The property range* is set.
2. The Animation.timeline becomes active

Meaning, when a timeline becomes active, any set range* property is resolved.
Or, when a range* property is set, if the timeline of the associated animation is active, that property is resolved.

[weinig]

This is not meant to be an issue about percentages, which seem to be well defined to happen at use time like animation-range-start/animation-range-end, but rather about things that get resolved normally for computed style, like font relative units (the em example) or sibling-index().

[ydaniv]

OK, but as I understood it you're specifically asking about the imperative interface, right? So can there be a different time to resolve these, other then what I suggested?