There's a very clear rule for it: if it contains a dash it's a custom element

That rule doesn't work in reverse. If there's no dash it could be either.

So now you've got to try to enforce some practice of using hyphens in all tag names that used to be class names, even if they're a single word that has no place for a hyphen?

It's getting even more confusing now, you see? Not less.

Just use classes. That's what they're there for.