Support for CSSOM CSSNamespaceRule interface

Source/core/css/CSSGroupingRule.cpp

Index: Source/core/css/CSSGroupingRule.cpp
diff --git a/Source/core/css/CSSGroupingRule.cpp b/Source/core/css/CSSGroupingRule.cpp
index 24f7147f6407a066f6618ece6732a452f8215773..f0b932981fa4ca93ae0dbdf528a0c98eea3766f7 100644
--- a/Source/core/css/CSSGroupingRule.cpp
+++ b/Source/core/css/CSSGroupingRule.cpp
@@ -71,13 +71,16 @@ unsigned CSSGroupingRule::insertRule(const String& ruleString, unsigned index, E
     CSSStyleSheet* styleSheet = parentStyleSheet();
     CSSParserContext context(parserContext(), UseCounter::getFrom(styleSheet));
     RefPtrWillBeRawPtr<StyleRuleBase> newRule = CSSParser::parseRule(context, styleSheet ? styleSheet->contents() : nullptr, ruleString);
-    // FIXME: @namespace rules have special handling in the CSSOM spec, but it
-    // mostly doesn't make sense since we don't support CSSNamespaceRule
-    if (!newRule || newRule->isNamespaceRule()) {
+    if (!newRule) {
         exceptionState.throwDOMException(SyntaxError, "the rule '" + ruleString + "' is invalid and cannot be parsed.");
         return 0;
     }
 
+    if (newRule->isNamespaceRule()) {
+        exceptionState.throwDOMException(HierarchyRequestError, "'@namespace' rules cannot be inserted inside a group rule.");
+        return 0;
+    }
+
     if (newRule->isImportRule()) {
         // FIXME: an HierarchyRequestError should also be thrown for a nested @media rule. They are
         // currently not getting parsed, resulting in a SyntaxError to get raised above.