Bug 1800112 - default-properties-on-the-math-root.html fails. r=emilio

This commit adds missing UA rules on the <math> root as specified
in https://w3c.github.io/mathml-core/#the-top-level-math-element

Differential Revision: https://phabricator.services.mozilla.com/D161815

layout/mathml/mathml.css

@@ -20,6 +20,8 @@ math {
   display: inline;
   font-size: inherit;
   font-style: normal;
+  font-weight: normal;
+  text-indent: 0;
   font-family: serif;
   line-height: normal;
   word-spacing: normal;

testing/web-platform/meta/mathml/relations/css-styling/default-properties-on-the-math-root.html.ini

@@ -1,8 +1,3 @@
 [default-properties-on-the-math-root.html]
   expected:
     if (os == "android") and fission: [OK, TIMEOUT]
-  [Value of text-indent on the <math> root]
-    expected: FAIL
-
-  [Value of font-weight on the <math> root]
-    expected: FAIL