[css3-transforms] 3D transform updates

Thanks for doing this! The spec is much improved.

Putting all non-3D content of a 3D rendering context at the very bottom of
z-order seems simpler and more robust than putting it at z=0 as the spec
currently says. Authors can usually opt into the z=0 effect by wrapping
content in a trivial 3D transform. There is one case where they can't: as
written, a preserve-3d child of the root element can be partially behind
and partially in front of a 'position:fixed' element. Is that something we
want to support --- or something we explicitly *don't* want to support?

Do we know anything about how Web-compatible the spec changes are?

Rob
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