style: Optimize all ids in the rightmost compound selector.
style: Optimize all ids in the rightmost compound selector.

This is effectively the same optimization Gecko has now.

MozReview-Commit-ID: 21HrjpHdpOz
Signed-off-by: Emilio Cobos Álvarez <86fa523305b8ad6a8a853c04cfef24a0445908d3@crisal.io>

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