Intent to prototype and ship: Make -- a reserved custom property name

[Emilio Cobos Álvarez]

Hi,

Summary:

In https://github.com/w3c/csswg-drafts/issues/2692 it was resolved that
-- should be an invalid custom property name. I plan to try aligning
with that change in bug 1467309.

There's some compat impact study looking at httparchive (thanks Lea!) in
https://discuss.httparchive.org/t/how-prevalent-are-empty-custom-properties-in-css/2153/2,
and I checked those pages and they seem like typos (the property is
defined, but never referenced, so there should be no rendering change).

So it seems worth a try.

Standard:
https://drafts.csswg.org/css-variables/#typedef-custom-property-name

Platform coverage: All

Preference: None

DevTools bug: N/A

Other browsers: We're the first ones in doing this, though I hope other
browsers would follow soon. I'll ping the relevant folks.

web-platform-tests: I changed the relevant WPTs in the patch.

-- Emilio