If you’ve checked out the 12 Days of Web (HT to @mandaris for sharing), and you were curious as to why popover is an attribute rather than a HTML tag, this section from OpenUI’s post about the feature explains the reason.

In short:

a content attribute allows behavior to be applied to any element

while maintaining semantics of that element:

Semantics are provided by elements, and behaviors are confered on those elements via attributes. This situation is exactly analogous to contenteditable or tabindex.

Yeah, okay. I can see why it was done that way.