As of November 2020, This works best on Chrome and Opera. Still not ready on Firefox.
To see if this has changed, take a look at Is Houdini Ready Yet.
For a tutoiral and explanation of how the paint API works, read "How to be Houdini and escape the limits of CSS" by Seth Corker on Benevolent Bytes to learn a little bit about the CSS Paint API.
A Demo of the CSS Paint API can be viewed in a browser that supports it.