It‘s used in Facebooks css. Remembered it from a nice article from Ahmad Shadeed. And while this limbo sure has some usefulness, it‘s way too obscure to use for the fun of it.

To add to this: CSS really has come a long way. This border-radius example can be done with Container Queries by now, which has quite good support already.

Lightning CSS is also great. A minifier at its core but also includes transpiling for older browser

Definitely not widely supported enough. Made the switch from sass back to css quite a while ago and let postcss polyfill less supported features like nesting.

avc1 is the name for H.264 (also called AVC) which is admittedly pretty confusing

MP4box has a great local web tool to test the mime type including the codecs parameter:

https://gpac.github.io/mp4box.js/test/filereader.html

Jake Archibald wrote a good article about this topic: https://jakearchibald.com/2022/html-codecs-parameter-for-av1/

Using a GitHub repository wiki is a low effort / zero cost option. https://docs.github.com/en/communities/documenting-your-project-with-wikis (Clearly not as charming as something self hosted though)