All of the frameworks are. They assume that vanilla HTML and the DOM API's can't do the job and they propose their own opinions as to how to "fix" it.
uibuilder has mostly already dealt with that, especially now that you can limit sent messages to specific client id's and/or page names (actually, currently everything is still send everywhere but the receiving client ignores it if not for them, this is something I want to enhance in the future by using socket.io's rooms features so as to be a bit more efficient and secure).
This is why the schema is a simplification of vanilla HMTL/DOM, it is inherently capable of accommodating future HTML/DOM features with minimal change and maximal backwards compatibility. This is largely because of the way that HTML and the DOM API's are constructed. HTML only allows a few interactions. For example, you can only add string attributes to HTML but you can add more complex properties via the DOM API's.
Is it really an extension? I don't believe it is (though perfectly possible I misinterpreted), it is an opinionated alternative to native HTML capabilities. It uses its own attribute types that are not native. Does it need to? What does that add and how does it have to bend HTML to accommodate those proprietary extensions? In the case of more complex frameworks, they have to generally take things out of the DOM in order to add their capabilities - maybe HTMX avoids that? But it still isn't standard, native HTML.
Yes, you would still have to take care of all the other things I mentioned that uibuilder has already taken care of.
Interesting discussion by the way. Keep it coming. 
