Fix erroroneous categories/parents for <style> in the element index

[ExplodingCabbage]

These were true when the index of elements was introduced, but obsoleted
when #318 removed the scoped
attribute.

Resolves #516

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Notes:

• This commit is an exact duplicate of whatwg/html@2acc100
• I've read https://github.com/w3c/html/blob/master/CONTRIBUTING.md and it sounds like I don't need to sign anything / declare anything / sacrifice any goats unless I'm making a "substantive contribution". I'm assuming that this doesn't count and that I can just open the PR without any other rituals involved; let me know if this is wrong.

[chaals]

@ExplodingCabbage You're right that for a change like this you don't need to sign anything.

But in the olden days one of the things you cuold do with CSS was to put it into an HTML document as flow content and then point to that document as a style sheet reference, and use that CSS in flow content as a set of style rules. In the 1990s that was considered a feature, and a useful one.

I'll check if it is still the case that you can do this - and if so, probably suggest you revert your whatwg change...

[chaals]

So I did some testing using an HTML document with some CSS inline as flow content as a stylesheet.

The results are slightly weird, but basically suggest that the whole thing still (mostly) works - and that the error introduced somewhere in history might have been in making
style {display:none} instead of distinguishing between when the style is in the head and when it's in the body, other than when it is scoped…

In a minute I'm going to close this PR and open an issue for now...