[css-animations-1] Use range notation for `<keyframe-selector>` as a percentage

[cdoublev]

<keyframe-selector> is defined as follows:

<keyframe-selector> = from | to | <percentage>

Values less than 0% or higher than 100% are invalid and cause their <keyframe-block> to be ignored.

I think it can be explicitly defined with the CSS bracketed range notation.

<keyframe-selector> = from | to | <percentage [0,100]>

[Loirooriol]

https://drafts.csswg.org/css-values-4/#numeric-ranges talks about values in properties, so maybe it should be generalized a bit.

[cdoublev]

Oh, that is right.

Actually, I did not want to create this issue because I have a list of those value definitions that can benefit from the range notation, and my intent was to report them in a single issue, as a reference (task list) for spec editors. It is noted that at the time of writing, the bracketed range notation is new; thus in most CSS specifications any range limitations are described only in prose. It just came out of my mind yesterday.

I hope you do not mind me closing this issue for now and recreating it later with a link to your comment on extending usage/definition of the range notation to CSS rules (maybe in Syntax).