I feel like the common case, especially for new HTML devs, is to use one website.

And when they get more experienced, they'll probably want to migrate that site to a new host (for example my site was bbkane.github.io before I moved it to bbkane.com), which will break absolute links to the first (now down) domain. Yes, best practice is to set up a redirect from old to new anyway, but that's the kind of thing someone doing this for the first time doesn't think about.

So there's definitely a case for beginners using relative links. And it's less repetitive to type!

I'm also interested in how it is going to be for multicultural individuals (with their own personal webpages). You have 2 webpages English, German. Could be a collection of these pages only in English.