Here’s a great talk by Das Surma where he looks into what Houdini is and how much of it is implemented in browsers. If you’re unfamiliar with that, Houdini is a series of technologies and APIs that gives developers low level access to how CSS properties work in a fundamental way. Check out Ana Tudor's deep dive into its impact on animations for some incredible examples of it in practice.
What I particularly like about this video is the way Das mentions the CSS Paint API which lets you do a bunch of bizarre things with CSS, such as creating "squircle" shapes and changing how borders work. It looks wonderfully robust and it should give us super powers in the near future. Ruth John wrote up this extensive overview on the topic earlier this year and it's worth a read as well.