Supported HTML/CSS/JS features depend on the client/OS you’re running Anki on. See for example:
If you know the browser, https://caniuse.com/ can serve as a lookup table.
AFAIK there’s no summary of currently used browser engines for all Anki clients. It’s a good idea though, and doesn’t seem that hard to implement.
Personally, I was positively surprised by Anki’s potiential as a JS learning environment. Working on things you actually make use of tends to be a very efficient way to learn new programming languages.