[mathml] Add stylesheet rule for semantics tag

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}

third_party/blink/renderer/core/css/mathml.css

@@ -56,8 +56,13 @@ math[display="block"] {
     outline: auto 1px -webkit-focus-ring-color;
 }
 
-mphantom {
-    visibility: hidden;
+mspace {
+    overflow: hidden !important;
+}
+
+/* <mrow>-like elements */
+semantics > :not(:first-child) {
+  display: none;
 }
 
 merror {
@@ -65,8 +70,8 @@ merror {
     background-color: lightYellow;
 }
 
-mspace {
-    overflow: hidden !important;
+mphantom {
+    visibility: hidden;
 }
 
 /* Token elements */

third_party/blink/web_tests/TestExpectations

@@ -1224,7 +1224,6 @@ crbug.com/6606 external/wpt/mathml/presentation-markup/fractions/frac-parameters
 crbug.com/6606 external/wpt/mathml/presentation-markup/fractions/frac-parameters-gap-006.html [ Failure ]
 crbug.com/6606 external/wpt/mathml/presentation-markup/mrow/inferred-mrow-stretchy.html [ Failure ]
 crbug.com/6606 external/wpt/mathml/presentation-markup/mrow/legacy-mrow-like-elements-001.html [ Failure ]
-crbug.com/6606 external/wpt/mathml/presentation-markup/mrow/legacy-mrow-like-elements-002.html [ Failure ]
 crbug.com/6606 external/wpt/mathml/presentation-markup/operators/embellished-operator-001.html [ Failure ]
 crbug.com/6606 external/wpt/mathml/presentation-markup/operators/embellished-operator-002.html [ Failure ]
 crbug.com/6606 external/wpt/mathml/presentation-markup/operators/mo-axis-height-1.html [ Failure ]
@@ -2658,7 +2657,6 @@ crbug.com/626703 external/wpt/infrastructure/testdriver/actions/crossOrigin.sub.
 crbug.com/626703 [ Mac11.0 ] external/wpt/html/semantics/embedded-content/media-elements/preserves-pitch.html [ Timeout ]
 crbug.com/626703 external/wpt/input-events/input-events-get-target-ranges-during-and-after-dispatch.tentative.html [ Timeout ]
 crbug.com/626703 [ Mac11.0 ] external/wpt/scroll-to-text-fragment/redirects.html [ Timeout ]
-crbug.com/626703 [ Linux ] external/wpt/mathml/presentation-markup/operators/operator-dictionary-symmetric-005.html [ Failure Crash ]
 crbug.com/626703 external/wpt/input-events/input-events-get-target-ranges-non-collapsed-selection.tentative.html?Backspace [ Timeout ]
 crbug.com/626703 external/wpt/input-events/input-events-get-target-ranges-non-collapsed-selection.tentative.html?TypingA [ Timeout ]
 crbug.com/626703 external/wpt/input-events/input-events-get-target-ranges-non-collapsed-selection.tentative.html?Delete [ Timeout ]