This is a really good primer on all the pieces that make up the Houdini approach to CSS—giving authors access to low-level APIs for rendering.
As is often repeated here, it’s still early days and caution is advised, but it’s still a good idea to wrap your head around what’s coming down the standards pipe.
There’s even more specs in Houdini’s list of drafts, but the future of those is rather uncertain and they are not much more than a placeholder for an idea at this point. Examples include custom overflow behaviors, CSS syntax extension API, extension of native scroll behavior and similar ambitious things that allow you to screw up your web app even worse than ever before. Rejoice!