Implement ::backdrop pseudo-element parsing

[nicoburns]

Context

• Fixes Style resolution issues on Github.com servo#34820

Notes

Github.com sets it's root-level CSS variables using a selector that includes a ::backdrop pseudo selector. It's not quite this, but it's morally equivalent to html, html ::backdrop. And the presence of ::backdrop causes the entire selector to fail to parse meaning that the variables never apply and styling is way off.

Changing the selector to html, html ::non-existent-selector causes the styles to fail to apply in Chrome too. So I think the strict parsing is correct per the spec. So I believe the only way to make this work is to implement parsing for ::backdrop. I don't think we support creating a backdrop in a current Servo, but if we did support then we probably would support matching styles against it, so enabling the selector seems reasonable to me.

Feedback wanted

I wasn't sure if it should be a Lazy pseudo-element. But Lazy is documented as being for regular but rarely used pseudo-elements which seemed to fit, whereas PreComputed is documented as being only for internal pseudo-elements which didn't seem to fit.

[Loirooriol]

I think we don't support modal dialogs nor fullscreen, so it shouldn't be observable whether we support ::backdrop or not, thus claiming we do isn't wrong :P

Not much sure about the pseudo cascade type either, but lazy sounds reasonable.

[jdm]

I think we don't support modal dialogs nor fullscreen, so it shouldn't be observable whether we support ::backdrop or not, thus claiming we do isn't wrong :P

Not much sure about the pseudo cascade type either, but lazy sounds reasonable.

We do support the DOM full screen API, unless you're referring to something else.

[nicoburns]

Does the fullscreen API use ::backdrop? I thought it was just "top layer" that used backdrops. I wouldn't expect fullscreen to have a backdrop because, well, it takes up the whole screen so you won't be be able to see anything behind.

[Loirooriol]

Yes, the fullscreen API places elements in the top layer, in fact ::backdrop was initially defined in the fullscreen spec: https://fullscreen.spec.whatwg.org/#css-pe-backdrop