Because there are so many native elements to keep track of now.

If you see an unfamiliar tag that has a reasonably simple name, you simply don't know if it's native or for formatting.

That's confusing. It's taking two categories of things and mixing them so you can't tell them apart.

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

	▲	crazygringo 21 minutes ago \| parent [-]
		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.