Part of the reason is of course that HTML was originally a document language and is slowly evolving in being both that and an application language.
I’ve said it before and I’ll say it again: It shouldn’t be. HTML is fundamentally a language for displaying information. Accepting input should not be a part of HTML because a) It’s out of scope, and b) it’s useful in places that HTML isn’t. Manipulating information is also out of scope (and thankfully has never been mushed in). An application requires bits from all three: Accepting, manipulating, and displaying (it also benefits from a fourth: styling).
Yeah, well, deal with it. Regarding b), what stops you from using it in other places?
Nothing actually stops it from being used. It just makes a lot more sense to have two separate specs instead mushing them together and then pulling them apart each time you need only one.
And that section of WA1 hasn’t been written yet, so there could potentially be a problem: If the
formelement is specified as only valid in certain contexts, or only allowing certain contents (like it was in HTML 4/XHTML 1, and like many things in WA1 have been), then you basically have to include an entire html document to get your form.So what in HTML5 is specified doesn’t limit you to make richer content models in other languages. I guess each HTML5 element (apart perhaps from tables because of layout complexity) allow all kinds of namespaced elements to be inserted. What that means however, is undefined and is more up to the language that reuses HTML5 to define.