Add <form> element

[petele]

Used compute_from: html.elements.form

[foolip]

This is a bit similar to <table> in that it's a collection of elements, but unlike <table> some of the elements can be used outside of <form>, like <button>.

@petele if we called this "Forms" like "Tables", are there any additional bits you think should be included, that don't make sense as standalone features?

Also paging @captainbrosset @ddbeck for thoughts on feature organization.

[petele]

I'll advocate for creating a separate feature for each of the <form> related elements.

• It would be a large feature with 11 elements, and many APIs included (leaving out <button>, including <form>)
• Most of the features landed at the very early, but some are significantly later, for example <datalist> which is Chrome 69. If there's a new form element in the future, it'll need its own entry.
• We may want to split <input> into the different types of input it supports, eg search, date, password, etc
• <button> would logically go in here, but can be used outside forms, so it would be in an awkward place
• We are already tagging these as group: forms

[ddbeck]

What about splitting the difference a little bit? I could imagine some sort foundational Forms feature consisting of a really, really stripped down thing:

• html.elements.form and descendants
• api.HTMLFormElement and descendants
• html.elements.input.type_text
• html.elements.input.type_submit

It'd be part of the forms group, but it'd let us be free to handle the various input types as separate features.

[petele]

I'm not yet convinced combining them is the way to go. Including some keys but not others feels very arbitrary, and adds complexity about where someone might find something. Is it in the core Form feature, or is it in the element?

[ddbeck]

This looks good. Safari showing DOM API support before the element itself is a bit weird. I filed mdn/browser-compat-data#24135 as an experiment. Hoping that Florian et al. have some feedback on whether I/we should be doing that routinely.

[ddbeck]

Also, for posterity: I haven't changed my mind exactly on the organization here, but I do feel like we might compose a feature, as suggested in #971. I think it'll be easier to sort out when we have all of the forms stuff complete.