[css-animations-2][scroll-animations-1] Avoid including properties that should be interpolated at boundaries when adding neutral keyframes

[kevers-google]

Steps 7 and 8 in https://drafts.csswg.org/css-animations-2/#keyframes outline the procedure for adding initial and final keyframes for properties that are missing at the boundaries. This procedure should be updated such that properties that are missing at 0 or 1, but which should be interpolated at the boundary are not included.

The set of "from" properties should include all properties found in a keyframe with offset <= 0.
Conversely, the set of "to" properties should include all properties found in a keyframe with offset >= 0.

Keyframes with a timeline offsets can result in computed offsets being outside the 0 to 1 range and should be interpolated at the boundary (see #7825).

[flackr]

I think this is already covered by the patch to the spec in scroll-animations-1 Appendix Named Timeline Range Keyframe Selectors where it says:

in these cases the automatic from (0%) and to (100%) keyframes are only generated for properties that don’t have keyframes at or earlier than 0% or at or after 100% (respectively).

[kevers-google]

Ah perfect. Didn't check there.