Intent to prototype: CSS zoom

[Emilio Cobos Álvarez]

Summary: Try to implement the IE CSS zoom property, while removing
-moz-transform prefixes (see discussion in the bug as for why), in order
to fix this long-standing interop issue for us (and the web more generally).

It might be the case that we cannot ship it, in which case this work
would inform the CSS Working Group of a better approach.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=390936
Specification: None (I know..)
Standards Body: The CSSWG showed interest in standardizing this in [1]
Platform coverage: All
Preference: layout.css.zoom.enabled
DevTools bug: I think no work should be needed for the most part, since
it gets reflected on existing APIs.
Link to standards-position discussion: No standard, though maybe we
could add one?
Other browsers: This is an IEism that WebKit and Blink implement since
forever.
web-platform-tests: I'm adding tentative WPTs as I go. Some basic ones
already in bug 1854441.

-- Emilio

[1]: https://github.com/w3c/csswg-drafts/issues/9397