This is a page from the Cascading Style Sheets Working Group Blog. Some other places to find information are the “current work” page, the www-style mailing list, the Future of CSS syndicator, and the issue list on Github.
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-path in the Masking spec, mark integration with clip as an issue
rotate3d() is interpolated by its angle when the two (normalized) vectors match, by matrix interpolation otherwise.
rotateX/Y/Z() interpolate with rotate3d() in the obvious way.
mask-attachment from the Masking spec.
mask-origin in the Masking spec.
mask-clip, and whether/how to allow unclipped masks.