Will animation composition be supported?

[hgl]

It doesn't seem to be in the Build Target Comparison table, but looks like something polyfillable.

[dstoc]

This is covered by "Additive animation" in the table, it will be implemented eventually.

[shans]

Just a note - additive animation is now slated for level 1 of the spec and hence will eventually be implemented in web-animations-1.