I wonder about the use of a non-standard HTML tag, . It works for styling purpose, and personally I don't have an issue with it - but it may put off some people due to it being "invalid" HTML..?

Yeah I don't get the point of that. Why not just a class name on the anchor you need anyway?

I can see the novelty of it ("Look at my fancy tag!"), but it would definitely be more helpful if it used a standard HTML element (preferably
). I've personally opted for this implementation on my own projects: https://github.com/tholman/github-corners, which are just SVGs which makes it super portable.