[css-transforms-2] Interpolation between none and a 3d rotate for rotate property

[BorisChiou]

About interpolation between none and a 3d rotate, the spec [1] only says:

When translate, rotate or scale are animating or transitioning, and the from value or to value (but not both) is none, the value none is replaced by the equivalent identity value (0px for translate, 0deg for rotate, 1 for scale).

This means we should replace the none as rotate: 0deg. This is fine for 2d rotation. However, the spec doesn't mention if the other value is a 3d rotation. e.g. from none to rotate: 7 8 9 100deg. The current spec might mislead people to replace none with rotate: 0 0 1 0deg (which is the most similar one to rotate: 0deg). However, for 3d rotation interpolation, it may make more sense to replacenone to rotate 7 8 9 0deg. (Maybe I am wrong.) In other words, I think it's better to note this: we should replace none with an identity value with a suitable rotate axis?

[1] https://drafts.csswg.org/css-transforms-2/#valdef-translate-none

[BorisChiou]

The reference wpt: https://github.com/web-platform-tests/wpt/blob/b207ce69984b926bf70fa7b4ad73fc1a0d7e0501/css/css-transforms/animation/rotate-interpolation.html#L41-L52