CSS Houdini is a multi-vendor effort to make more aspects of CSS layout scriptable rather than all having to be specified ahead of time.

https://ishoudinireadyyet.com/

This gives a much greater ability to iterate rapidly on new layout and styling details, polyfill for specifications that aren't widely implemented yet, and hook into some parts of the browser stack that you previously just had to use as is and trust that if you need a new feature, it would eventually be specified and/or implemented.

I think this article does the best job of explaining the whole effort:

https://www.smashingmagazine.com/2016/03/houdini-maybe-the-m...