Reject compound selectors with unknown namespace prefixes

third_party/WebKit/Source/core/css/parser/CSSSelectorParser.cpp

Index: third_party/WebKit/Source/core/css/parser/CSSSelectorParser.cpp
diff --git a/third_party/WebKit/Source/core/css/parser/CSSSelectorParser.cpp b/third_party/WebKit/Source/core/css/parser/CSSSelectorParser.cpp
index 076f85f14eb4f2913a189c91955f70fb1c273c62..46a23a0470cfaefae476abaed0b39c6b75822a69 100644
--- a/third_party/WebKit/Source/core/css/parser/CSSSelectorParser.cpp
+++ b/third_party/WebKit/Source/core/css/parser/CSSSelectorParser.cpp
@@ -572,8 +572,10 @@ void CSSSelectorParser::prependTypeSelectorIfNeeded(const AtomicString& namespac
 
     AtomicString determinedElementName = elementName.isNull() ? starAtom : elementName;
     AtomicString namespaceURI = determineNamespace(namespacePrefix);
-    if (namespaceURI.isNull())
+    if (namespaceURI.isNull()) {
+        m_failedParsing = true;
         return;
+    }
     QualifiedName tag = QualifiedName(namespacePrefix, determinedElementName, namespaceURI);
 
     if (compoundSelector->needsImplicitShadowCrossingCombinatorForMatching())