Slightly less well-known than XDG directories among the developers of Linux-targeted software, it would seem.

Seriously, what an oxymoronic name. "/index.html" is a well known URL, literally: most of web-developers are aware of it. But inventing a bunch of URLs with predefined semantics and then slapping the "well-known" label on it... well, it won't magically make them actually well-known.