Copy the DOM XPath interfaces from the WHATWG wiki

[foolip]

https://wiki.whatwg.org/wiki/DOM_XPath

Related: #67

---

Preview | Diff

[foolip]

Tests using this IDL already exist:
https://wpt.fyi/results/domxpath/interfaces.tentative.html?run_id=258200004&run_id=264060011&run_id=250500005&run_id=266120012

They show that all of this is already implemented, the failures in Edge and Safari are because of minor differences in length properties, what exception is thrown, and so on.

[gsnedders]

There's more changes than just that, some of them simplify the API by making arguments optional.

Let's also add something, possibly just normative, saying:

• XPathException is gone (just use DOMException)
• The query is matched against the DOM, and therefore contrary to the XPath 1.0 data model the root element has a parent (the Document) and text nodes can be adjacent to one another. [wilful violation]

[annevk]

Let me know if you want to improve this further or have me do it, but there's two changes I'd like to see here before merging this:

1. The first paragraph should be reworded and flagged as an issue (using class=issue), ideally linking to an open issue in the DOM Standard that would allow for someone to take on the work of defining the API in more detail. Sam's points can be included in that issue description. I don't think we should have normative wording outside of the issue.
2. The note should probably be reworded a bit to be slightly more formal.

Also, if anyone has issues with merging this, please speak up, as I don't have a good reason to not do this at this point.

[foolip]

I'll polish this up next week.

[foolip]

I didn't get to this this week and I'm OOO tomorrow. Snoozing it until next week but won't be offended if someone takes it over.

[foolip]

@annevk I pushed the changes. However, class=issue doesn't seem to be a thing, so I used class=warning. Don't love the way it looks, is there a better class?

[zcorpan]

@foolip class=XXX

[foolip]

@zcorpan thanks, trying that instead

[annevk]

This is blocked by w3ctag/promises-guide#27 for now. I adjusted the wording a bit to make it clear that defining these is still a goal, albeit perhaps with less priority than some would like.

[foolip]

w3ctag/promises-guide#27 is resolved, Travis now happy.