Do you want to know how the CSS WG works? Fantasai has written about:csswg, An Inside View of the CSS Working Group at W3C.
clip-pathin the Masking spec, mark integration with
clipas an issue
rotate3d()is interpolated by its angle when the two (normalized) vectors match, by matrix interpolation otherwise.
rotate3d()in the obvious way.
mask-attachmentfrom the Masking spec.
mask-originin the Masking spec.
mask-clip, and whether/how to allow unclipped masks.