Latest CSS Houdini spec drafts: https://drafts.css-houdini.org/ (http://dev.w3.org/houdini/ redirects there)
Here are a couple of some direct links to specs:
* [https://drafts.css-houdini.org/css-typed-om/ CSS Typed OM Level 1]
* [https://drafts.css-houdini.org/worklets/ Worklets Level 1] (needed for Paint)