fix typo that caused the CSS declaration to be ignored -- comments in CSS must be delimited with /* */ and not //

layout/mathml/content/src/mathml.css

@@ -298,7 +298,7 @@ mtd {
   white-space: nowrap;
 }
 
-// Don't support mtr without mtable, nor mtd without mtr
+/* Don't support mtr without mtable, nor mtd without mtr */
 :not(mtable) > mtr,
 :not(mtr) > mtd {
   display: none !important;