[scroll-animations-1] Ambiguous grammar of animation-range

[cdoublev]

https://drafts.csswg.org/scroll-animations-1/#animation-range

I assume animation-range: entry 10% should expand to animation-range-start: entry 10% and animation-range-end: normal (see next comment) animation-range-end: entry 100%.

But assuming the grammar can be symbolized with [a | b a?]{1,2} and the input is b a, I do not know which spec defines the matching behavior that says how many repetitions should match the input.

Maybe it should be clarified in prose (and the examples could include this case). For example:

If only a <timeline-range-name> and a <length-percentage> is specified, it is assumed to match <'animation-range-start'>, and <'animation-range-end'> is omitted.

Note this also means that when serializing animation-range, 0% cannot be omitted for the <timeline-range-name> of animation-range-start if animation-range-end is declared with a (single) <length-percentage>.

[johannesodland]

I stumbled over the same issue and think it would be helpful to clarify this ambiguity in the spec.

I have one question regarding the expansion though @cdoublev:

If entry 10% is consumed by <'animation-range-start'>, shouldn't animation-range: entry 10% expand to animation-range-start: entry 10% and animation-range-end: entry 100% due to how animation-range-end is expanded when omitted?

If <'animation-range-end'> is omitted and <'animation-range-start'> includes a component, then animation-range-end is set to that same and 100%.

[cdoublev]

Oh yes, you are absolutely right.