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The semantics tag should hide children except the first one [1],
so add a stylesheet rule for that.
This CL also reorganizes the stylesheet to match the order
in the specification.

[1] https://mathml-refresh.github.io/mathml-core/#semantics-and-presentation

Bug: 6606
Change-Id: I8986f7953363e302e7c637bde1e5b67c3a040bef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2467903
Commit-Queue: Rob Buis <rbuis@igalia.com>
Reviewed-by: Frédéric Wang <fwang@igalia.com>
Cr-Commit-Position: refs/heads/master@{#816596}