diff --git a/parser/html/javasrc/TreeBuilder.java b/parser/html/javasrc/TreeBuilder.java
index 32973e535d4d..3db2c69aef44 100644
--- a/parser/html/javasrc/TreeBuilder.java
+++ b/parser/html/javasrc/TreeBuilder.java
@@ -868,6 +868,8 @@ public abstract class TreeBuilder<T> implements TokenHandler,
      */
     public final void characters(@Const @NoLength char[] buf, int start, int length)
             throws SAXException {
+        // Note: Can't attach error messages to EOF in C++ yet
+
         // CPPONLY: if (tokenizer.isViewingXmlSource()) {
         // CPPONLY: return;
         // CPPONLY: }
@@ -1271,6 +1273,7 @@ public abstract class TreeBuilder<T> implements TokenHandler,
 
     public final void eof() throws SAXException {
         flushCharacters();
+        // Note: Can't attach error messages to EOF in C++ yet
         eofloop: for (;;) {
             if (isInForeign()) {
                 // [NOCPP[
diff --git a/parser/html/nsHtml5Highlighter.cpp b/parser/html/nsHtml5Highlighter.cpp
index 7e1f3102c378..68832fdc3153 100644
--- a/parser/html/nsHtml5Highlighter.cpp
+++ b/parser/html/nsHtml5Highlighter.cpp
@@ -205,10 +205,10 @@ nsHtml5Highlighter::Transition(PRInt32 aState, bool aReconsume, PRInt32 aPos)
     case NS_HTML5TOKENIZER_TAG_NAME:
       switch (aState) {
         case NS_HTML5TOKENIZER_BEFORE_ATTRIBUTE_NAME:
-          EndInline(); // NS_HTML5TOKENIZER_TAG_NAME
+          EndSpanOrA(); // NS_HTML5TOKENIZER_TAG_NAME
           break;
         case NS_HTML5TOKENIZER_SELF_CLOSING_START_TAG:
-          EndInline(); // NS_HTML5TOKENIZER_TAG_NAME
+          EndSpanOrA(); // NS_HTML5TOKENIZER_TAG_NAME
           StartSpan(); // for highlighting the slash
           mSlash = CurrentNode();
           break;
@@ -235,10 +235,10 @@ nsHtml5Highlighter::Transition(PRInt32 aState, bool aReconsume, PRInt32 aPos)
       switch (aState) {
         case NS_HTML5TOKENIZER_AFTER_ATTRIBUTE_NAME:
         case NS_HTML5TOKENIZER_BEFORE_ATTRIBUTE_VALUE:
-          EndInline(); // NS_HTML5TOKENIZER_BEFORE_ATTRIBUTE_NAME
+          EndSpanOrA(); // NS_HTML5TOKENIZER_BEFORE_ATTRIBUTE_NAME
           break;
         case NS_HTML5TOKENIZER_SELF_CLOSING_START_TAG:
-          EndInline(); // NS_HTML5TOKENIZER_BEFORE_ATTRIBUTE_NAME
+          EndSpanOrA(); // NS_HTML5TOKENIZER_BEFORE_ATTRIBUTE_NAME
           StartSpan(); // for highlighting the slash
           mSlash = CurrentNode();
           break;
@@ -266,7 +266,7 @@ nsHtml5Highlighter::Transition(PRInt32 aState, bool aReconsume, PRInt32 aPos)
     case NS_HTML5TOKENIZER_ATTRIBUTE_VALUE_SINGLE_QUOTED:
       switch (aState) {
         case NS_HTML5TOKENIZER_AFTER_ATTRIBUTE_VALUE_QUOTED:
-          EndInline();
+          EndSpanOrA();
           break;
         case NS_HTML5TOKENIZER_CONSUME_CHARACTER_REFERENCE:
           StartSpan();
@@ -292,7 +292,7 @@ nsHtml5Highlighter::Transition(PRInt32 aState, bool aReconsume, PRInt32 aPos)
       }
       break;
     case NS_HTML5TOKENIZER_SELF_CLOSING_START_TAG:
-      EndInline(); // end the slash highlight
+      EndSpanOrA(); // end the slash highlight
       switch (aState) {
         case NS_HTML5TOKENIZER_BEFORE_ATTRIBUTE_NAME:
           break;
@@ -304,7 +304,7 @@ nsHtml5Highlighter::Transition(PRInt32 aState, bool aReconsume, PRInt32 aPos)
     case NS_HTML5TOKENIZER_ATTRIBUTE_VALUE_UNQUOTED:
       switch (aState) {
         case NS_HTML5TOKENIZER_BEFORE_ATTRIBUTE_NAME:
-          EndInline();
+          EndSpanOrA();
           break;
         case NS_HTML5TOKENIZER_CONSUME_CHARACTER_REFERENCE:
           StartSpan();
@@ -353,14 +353,14 @@ nsHtml5Highlighter::Transition(PRInt32 aState, bool aReconsume, PRInt32 aPos)
       }
       break;
     case NS_HTML5TOKENIZER_CONSUME_CHARACTER_REFERENCE:
-      EndInline(); // the span for the ampersand
+      EndSpanOrA(); // the span for the ampersand
       switch (aState) {
         case NS_HTML5TOKENIZER_CONSUME_NCR:
         case NS_HTML5TOKENIZER_CHARACTER_REFERENCE_HILO_LOOKUP:
           break;
         default:
           // not actually a character reference
-          EndInline();
+          EndSpanOrA();
           break;
       }
       break;
@@ -369,13 +369,13 @@ nsHtml5Highlighter::Transition(PRInt32 aState, bool aReconsume, PRInt32 aPos)
         break;
       }
       // not actually a character reference
-      EndInline();
+      EndSpanOrA();
       break;
     case NS_HTML5TOKENIZER_CHARACTER_REFERENCE_TAIL:
       if (!aReconsume) {
         FlushCurrent();
       }
-      EndInline();
+      EndSpanOrA();
       break;
     case NS_HTML5TOKENIZER_DECIMAL_NRC_LOOP:
     case NS_HTML5TOKENIZER_HEX_NCR_LOOP:
@@ -388,7 +388,7 @@ nsHtml5Highlighter::Transition(PRInt32 aState, bool aReconsume, PRInt32 aPos)
           AddClass(sEntity);
           break;
       }
-      EndInline();
+      EndSpanOrA();
       break;
     case NS_HTML5TOKENIZER_CLOSE_TAG_OPEN:
       switch (aState) {
@@ -406,18 +406,18 @@ nsHtml5Highlighter::Transition(PRInt32 aState, bool aReconsume, PRInt32 aPos)
         StartSpan(); // don't know if it is "end-tag" yet :-(
         break;
       }
-      EndInline();
+      EndSpanOrA();
       StartCharacters();
       break;
     case NS_HTML5TOKENIZER_NON_DATA_END_TAG_NAME:
       switch (aState) {
         case NS_HTML5TOKENIZER_BEFORE_ATTRIBUTE_NAME:
           AddClass(sEndTag);
-          EndInline();
+          EndSpanOrA();
           break;
         case NS_HTML5TOKENIZER_SELF_CLOSING_START_TAG:
           AddClass(sEndTag);
-          EndInline();
+          EndSpanOrA();
           StartSpan(); // for highlighting the slash
           mSlash = CurrentNode();
           break;
@@ -557,7 +557,7 @@ nsHtml5Highlighter::StartSpan(const PRUnichar* aClass)
 }
 
 void
-nsHtml5Highlighter::EndInline()
+nsHtml5Highlighter::EndSpanOrA()
 {
   FlushChars();
   Pop();
@@ -596,10 +596,10 @@ void
 nsHtml5Highlighter::FinishTag()
 {
   while (mInlinesOpen > 1) {
-    EndInline();
+    EndSpanOrA();
   }
   FlushCurrent(); // >
-  EndInline(); // DATA
+  EndSpanOrA(); // DATA
   NS_ASSERTION(!mInlinesOpen, "mInlinesOpen got out of sync!");
   StartCharacters();
 }
@@ -685,14 +685,14 @@ void
 nsHtml5Highlighter::MaybeLinkifyAttributeValue(nsHtml5AttributeName* aName,
                                                nsString* aValue)
 {
-  if (!(nsHtml5AttributeName::ATTR_HREF == aName
-      || nsHtml5AttributeName::ATTR_SRC == aName
-      || nsHtml5AttributeName::ATTR_ACTION == aName
-      || nsHtml5AttributeName::ATTR_CITE == aName
-      || nsHtml5AttributeName::ATTR_BACKGROUND == aName
-      || nsHtml5AttributeName::ATTR_LONGDESC == aName
-      || nsHtml5AttributeName::ATTR_XLINK_HREF == aName
-      || nsHtml5AttributeName::ATTR_DEFINITIONURL == aName)) {
+  if (!(nsHtml5AttributeName::ATTR_HREF == aName ||
+        nsHtml5AttributeName::ATTR_SRC == aName ||
+        nsHtml5AttributeName::ATTR_ACTION == aName ||
+        nsHtml5AttributeName::ATTR_CITE == aName ||
+        nsHtml5AttributeName::ATTR_BACKGROUND == aName ||
+        nsHtml5AttributeName::ATTR_LONGDESC == aName ||
+        nsHtml5AttributeName::ATTR_XLINK_HREF == aName ||
+        nsHtml5AttributeName::ATTR_DEFINITIONURL == aName)) {
     return;
   }
   AddViewSourceHref(*aValue);
diff --git a/parser/html/nsHtml5Highlighter.h b/parser/html/nsHtml5Highlighter.h
index 3def948c9d5d..b2b06b22e245 100644
--- a/parser/html/nsHtml5Highlighter.h
+++ b/parser/html/nsHtml5Highlighter.h
@@ -192,9 +192,9 @@ class nsHtml5Highlighter
     void StartSpan(const PRUnichar* aClass);
 
     /**
-     * End the current <span> or <a>.
+     * End the current <span> or <a> in the highlighter output.
      */
-    void EndInline();
+    void EndSpanOrA();
 
     /**
      * Starts a wrapper around a run of characters.
@@ -222,8 +222,10 @@ class nsHtml5Highlighter
     void FlushCurrent();
 
     /**
-     * Finishes a source tag being highlighted by closing the open <span> and
-     * <a> elements.
+     * Finishes highlighting a tag in the input data by closing the open
+     * <span> and <a> elements in the highlighter output and then starts
+     * another <span> for potentially highlighting characters potentially
+     * appearing next.
      */
     void FinishTag();
 
@@ -238,6 +240,9 @@ class nsHtml5Highlighter
     /**
      * Allocates a handle for an element.
      *
+     * See the documentation for nsHtml5TreeBuilder::AllocateContentHandle()
+     * in nsHtml5TreeBuilderHSupplement.h.
+     *
      * @return the handle
      */
     nsIContent** AllocateContentHandle();
diff --git a/parser/html/nsHtml5StreamParser.cpp b/parser/html/nsHtml5StreamParser.cpp
index f91fa98c4788..6b569f5b27a8 100644
--- a/parser/html/nsHtml5StreamParser.cpp
+++ b/parser/html/nsHtml5StreamParser.cpp
@@ -516,6 +516,14 @@ nsHtml5StreamParser::FinalizeSniffing(const PRUint8* aFromSegment, // can be nul
     XML_SetUserData(ud.mExpat, static_cast<void*>(&ud));
 
     XML_Status status = XML_STATUS_OK;
+
+    // aFromSegment points to the data obtained from the current network
+    // event. mSniffingBuffer (if it exists) contains the data obtained before
+    // the current event. Thus, mSniffingLenth bytes of mSniffingBuffer
+    // followed by aCountToSniffingLimit bytes from aFromSegment are the
+    // first 1024 bytes of the file (or the file as a whole if the file is
+    // 1024 bytes long or shorter). Thus, we parse both buffers, but if the
+    // first call succeeds already, we skip parsing the second buffer.
     if (mSniffingBuffer) {
       status = XML_Parse(ud.mExpat,
                          reinterpret_cast<const char*>(mSniffingBuffer.get()),
@@ -682,8 +690,8 @@ nsHtml5StreamParser::SniffStreamBytes(const PRUint8* aFromSegment,
     PRUint32 countToSniffingLimit =
         NS_HTML5_STREAM_PARSER_SNIFFING_BUFFER_SIZE - mSniffingLength;
     if (mMode == NORMAL || mMode == VIEW_SOURCE_HTML) {
-      nsHtml5ByteReadable readable(aFromSegment, aFromSegment
-          + countToSniffingLimit);
+      nsHtml5ByteReadable readable(aFromSegment, aFromSegment +
+          countToSniffingLimit);
       mMetaScanner->sniff(&readable, getter_AddRefs(mUnicodeDecoder), mCharset);
       if (mUnicodeDecoder) {
         mUnicodeDecoder->SetInputErrorBehavior(
diff --git a/parser/html/nsHtml5Tokenizer.cpp b/parser/html/nsHtml5Tokenizer.cpp
index 6d120c94b69b..28428e320bcd 100644
--- a/parser/html/nsHtml5Tokenizer.cpp
+++ b/parser/html/nsHtml5Tokenizer.cpp
@@ -496,7 +496,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
                 state = P::transition(mViewSource, NS_HTML5TOKENIZER_PROCESSING_INSTRUCTION, reconsume, pos);
                 NS_HTML5_CONTINUE(stateloop);
               }
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errProcessingInstruction();
               }
               clearLongStrBufAndAppend(c);
@@ -504,7 +504,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               NS_HTML5_CONTINUE(stateloop);
             }
             case '>': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errLtGt();
               }
               tokenHandler->characters(nsHtml5Tokenizer::LT_GT, 0, 2);
@@ -513,7 +513,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               NS_HTML5_CONTINUE(stateloop);
             }
             default: {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errBadCharAfterLt(c);
               }
               tokenHandler->characters(nsHtml5Tokenizer::LT_GT, 0, 1);
@@ -617,7 +617,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
             case '\'':
             case '<':
             case '=': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errBadCharBeforeAttributeNameOrNull(c);
               }
             }
@@ -682,7 +682,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
             case '\"':
             case '\'':
             case '<': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errQuoteOrLtInAttributeNameOrNull(c);
               }
             }
@@ -734,7 +734,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               NS_HTML5_CONTINUE(stateloop);
             }
             case '>': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errAttributeValueMissing();
               }
               addAttributeWithoutValue();
@@ -750,7 +750,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
             case '<':
             case '=':
             case '`': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errLtOrEqualsOrGraveInUnquotedAttributeOrNull(c);
               }
             }
@@ -839,7 +839,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               NS_HTML5_CONTINUE(stateloop);
             }
             default: {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errNoSpaceBetweenAttributes();
               }
               state = P::transition(mViewSource, NS_HTML5TOKENIZER_BEFORE_ATTRIBUTE_NAME, reconsume, pos);
@@ -864,7 +864,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
             NS_HTML5_CONTINUE(stateloop);
           }
           default: {
-            if (P::viewingSource) {
+            if (P::reportErrors) {
               errSlashNotFollowedByGt();
             }
             state = P::transition(mViewSource, NS_HTML5TOKENIZER_BEFORE_ATTRIBUTE_NAME, reconsume, pos);
@@ -923,7 +923,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
             case '\'':
             case '=':
             case '`': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errUnquotedAttributeValOrNull(c);
               }
             }
@@ -977,7 +977,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
             case '\"':
             case '\'':
             case '<': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errQuoteOrLtInAttributeNameOrNull(c);
               }
             }
@@ -1021,7 +1021,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               }
             }
             default: {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errBogusComment();
               }
               clearLongStrBuf();
@@ -1049,7 +1049,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               NS_HTML5_BREAK(markupdeclarationhyphenloop);
             }
             default: {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errBogusComment();
               }
               state = P::transition(mViewSource, NS_HTML5TOKENIZER_BOGUS_COMMENT, reconsume, pos);
@@ -1073,7 +1073,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               NS_HTML5_CONTINUE(stateloop);
             }
             case '>': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errPrematureEndOfComment();
               }
               emitComment(0, pos);
@@ -1194,7 +1194,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               NS_HTML5_CONTINUE(stateloop);
             }
             case '!': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errHyphenHyphenBang();
               }
               appendLongStrBuf(c);
@@ -1261,7 +1261,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
             NS_HTML5_CONTINUE(stateloop);
           }
           case '>': {
-            if (P::viewingSource) {
+            if (P::reportErrors) {
               errPrematureEndOfComment();
             }
             emitComment(1, pos);
@@ -1298,7 +1298,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
             if (c == nsHtml5Tokenizer::CDATA_LSQB[index]) {
               appendLongStrBuf(c);
             } else {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errBogusComment();
               }
               state = P::transition(mViewSource, NS_HTML5TOKENIZER_BOGUS_COMMENT, reconsume, pos);
@@ -1474,7 +1474,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
             } else if (c >= 'A' && c <= 'Z') {
               firstCharKey = c - 'A';
             } else {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errNoNamedCharacterMatch();
               }
               emitOrAppendStrBuf(returnState);
@@ -1507,7 +1507,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
             }
           }
           if (!hilo) {
-            if (P::viewingSource) {
+            if (P::reportErrors) {
               errNoNamedCharacterMatch();
             }
             emitOrAppendStrBuf(returnState);
@@ -1578,7 +1578,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
         }
         outer_end: ;
         if (candidate == -1) {
-          if (P::viewingSource) {
+          if (P::reportErrors) {
             errNoNamedCharacterMatch();
           }
           emitOrAppendStrBuf(returnState);
@@ -1599,7 +1599,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
                 ch = strBuf[strBufMark];
               }
               if (ch == '=' || (ch >= '0' && ch <= '9') || (ch >= 'A' && ch <= 'Z') || (ch >= 'a' && ch <= 'z')) {
-                if (P::viewingSource) {
+                if (P::reportErrors) {
                   errNoNamedCharacterMatch();
                 }
                 appendStrBufToLongStrBuf();
@@ -1609,11 +1609,11 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               }
             }
             if ((returnState & NS_HTML5TOKENIZER_DATA_AND_RCDATA_MASK)) {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errUnescapedAmpersandInterpretedAsCharacterReference();
               }
             } else {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errNotSemicolonTerminated();
               }
             }
@@ -1690,7 +1690,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               state = P::transition(mViewSource, NS_HTML5TOKENIZER_HANDLE_NCR_VALUE, reconsume, pos);
               NS_HTML5_BREAK(decimalloop);
             } else {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errNoDigitsInNCR();
               }
               appendStrBuf(';');
@@ -1703,7 +1703,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
             }
           } else {
             if (!seenDigits) {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errNoDigitsInNCR();
               }
               emitOrAppendStrBuf(returnState);
@@ -1714,7 +1714,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               reconsume = true;
               NS_HTML5_CONTINUE(stateloop);
             } else {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errCharRefLacksSemicolon();
               }
               if (!(returnState & NS_HTML5TOKENIZER_DATA_AND_RCDATA_MASK)) {
@@ -1766,7 +1766,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               state = P::transition(mViewSource, NS_HTML5TOKENIZER_HANDLE_NCR_VALUE, reconsume, pos);
               NS_HTML5_CONTINUE(stateloop);
             } else {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errNoDigitsInNCR();
               }
               appendStrBuf(';');
@@ -1779,7 +1779,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
             }
           } else {
             if (!seenDigits) {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errNoDigitsInNCR();
               }
               emitOrAppendStrBuf(returnState);
@@ -1790,7 +1790,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               reconsume = true;
               NS_HTML5_CONTINUE(stateloop);
             } else {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errCharRefLacksSemicolon();
               }
               if (!(returnState & NS_HTML5TOKENIZER_DATA_AND_RCDATA_MASK)) {
@@ -1839,7 +1839,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
         c = checkChar(buf, pos);
         switch(c) {
           case '>': {
-            if (P::viewingSource) {
+            if (P::reportErrors) {
               errLtSlashGt();
             }
             cstart = pos + 1;
@@ -1848,7 +1848,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
           }
           case '\r': {
             silentCarriageReturn();
-            if (P::viewingSource) {
+            if (P::reportErrors) {
               errGarbageAfterLtSlash();
             }
             clearLongStrBufAndAppend('\n');
@@ -1857,7 +1857,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
           }
           case '\n': {
             silentLineFeed();
-            if (P::viewingSource) {
+            if (P::reportErrors) {
               errGarbageAfterLtSlash();
             }
             clearLongStrBufAndAppend('\n');
@@ -1877,7 +1877,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               state = P::transition(mViewSource, NS_HTML5TOKENIZER_TAG_NAME, reconsume, pos);
               NS_HTML5_CONTINUE(stateloop);
             } else {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errGarbageAfterLtSlash();
               }
               clearLongStrBufAndAppend(c);
@@ -2625,7 +2625,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
             if (folded == nsHtml5Tokenizer::OCTYPE[index]) {
               appendLongStrBuf(c);
             } else {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errBogusComment();
               }
               state = P::transition(mViewSource, NS_HTML5TOKENIZER_BOGUS_COMMENT, reconsume, pos);
@@ -2669,7 +2669,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               NS_HTML5_BREAK(doctypeloop);
             }
             default: {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errMissingSpaceBeforeDoctypeName();
               }
               state = P::transition(mViewSource, NS_HTML5TOKENIZER_BEFORE_DOCTYPE_NAME, reconsume, pos);
@@ -2704,7 +2704,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               continue;
             }
             case '>': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errNamelessDoctype();
               }
               forceQuirks = true;
@@ -2868,7 +2868,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               NS_HTML5_BREAK(afterdoctypepublickeywordloop);
             }
             case '\"': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errNoSpaceBetweenDoctypePublicKeywordAndQuote();
               }
               clearLongStrBuf();
@@ -2876,7 +2876,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               NS_HTML5_CONTINUE(stateloop);
             }
             case '\'': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errNoSpaceBetweenDoctypePublicKeywordAndQuote();
               }
               clearLongStrBuf();
@@ -2884,7 +2884,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               NS_HTML5_CONTINUE(stateloop);
             }
             case '>': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errExpectedPublicId();
               }
               forceQuirks = true;
@@ -2931,7 +2931,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               NS_HTML5_CONTINUE(stateloop);
             }
             case '>': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errExpectedPublicId();
               }
               forceQuirks = true;
@@ -2961,7 +2961,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               NS_HTML5_BREAK(doctypepublicidentifierdoublequotedloop);
             }
             case '>': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errGtInPublicId();
               }
               forceQuirks = true;
@@ -3016,7 +3016,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               NS_HTML5_CONTINUE(stateloop);
             }
             case '\"': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errNoSpaceBetweenPublicAndSystemIds();
               }
               clearLongStrBuf();
@@ -3024,7 +3024,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               NS_HTML5_CONTINUE(stateloop);
             }
             case '\'': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errNoSpaceBetweenPublicAndSystemIds();
               }
               clearLongStrBuf();
@@ -3096,7 +3096,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               NS_HTML5_CONTINUE(stateloop);
             }
             case '>': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errGtInSystemId();
               }
               forceQuirks = true;
@@ -3239,7 +3239,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               NS_HTML5_BREAK(afterdoctypesystemkeywordloop);
             }
             case '\"': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errNoSpaceBetweenDoctypeSystemKeywordAndQuote();
               }
               clearLongStrBuf();
@@ -3247,7 +3247,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               NS_HTML5_CONTINUE(stateloop);
             }
             case '\'': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errNoSpaceBetweenDoctypeSystemKeywordAndQuote();
               }
               clearLongStrBuf();
@@ -3255,7 +3255,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               NS_HTML5_CONTINUE(stateloop);
             }
             case '>': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errExpectedPublicId();
               }
               forceQuirks = true;
@@ -3302,7 +3302,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               NS_HTML5_BREAK(beforedoctypesystemidentifierloop);
             }
             case '>': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errExpectedSystemId();
               }
               forceQuirks = true;
@@ -3332,7 +3332,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               NS_HTML5_CONTINUE(stateloop);
             }
             case '>': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errGtInSystemId();
               }
               forceQuirks = true;
@@ -3372,7 +3372,7 @@ nsHtml5Tokenizer::stateLoop(PRInt32 state, PRUnichar c, PRInt32 pos, PRUnichar*
               NS_HTML5_CONTINUE(stateloop);
             }
             case '>': {
-              if (P::viewingSource) {
+              if (P::reportErrors) {
                 errGtInPublicId();
               }
               forceQuirks = true;
diff --git a/parser/html/nsHtml5TokenizerCppSupplement.h b/parser/html/nsHtml5TokenizerCppSupplement.h
index e62481b38566..9b71c6db2bfe 100644
--- a/parser/html/nsHtml5TokenizerCppSupplement.h
+++ b/parser/html/nsHtml5TokenizerCppSupplement.h
@@ -64,67 +64,79 @@ nsHtml5Tokenizer::errWarnLtSlashInRcdata()
 {
 }
 
+// The null checks below annotated NS_LIKELY are not actually necessary.
+
 void
 nsHtml5Tokenizer::errUnquotedAttributeValOrNull(PRUnichar c)
 {
-  switch (c) {
-    case '<':
-      mViewSource->AddErrorToCurrentNode("errUnquotedAttributeLt");
-      return;
-    case '`':
-      mViewSource->AddErrorToCurrentNode("errUnquotedAttributeGrave");
-      return;
-    case '\'':
-    case '"':
-      mViewSource->AddErrorToCurrentNode("errUnquotedAttributeQuote");
-      return;
-    case '=':
-      mViewSource->AddErrorToCurrentNode("errUnquotedAttributeEquals");
-      return;
+  if (NS_LIKELY(mViewSource)) {
+    switch (c) {
+      case '<':
+        mViewSource->AddErrorToCurrentNode("errUnquotedAttributeLt");
+        return;
+      case '`':
+        mViewSource->AddErrorToCurrentNode("errUnquotedAttributeGrave");
+        return;
+      case '\'':
+      case '"':
+        mViewSource->AddErrorToCurrentNode("errUnquotedAttributeQuote");
+        return;
+      case '=':
+        mViewSource->AddErrorToCurrentNode("errUnquotedAttributeEquals");
+        return;
+    }
   }
 }
 
 void
 nsHtml5Tokenizer::errLtOrEqualsOrGraveInUnquotedAttributeOrNull(PRUnichar c)
 {
-  switch (c) {
-    case '=':
-      mViewSource->AddErrorToCurrentNode("errUnquotedAttributeStartEquals");
-      return;
-    case '<':
-      mViewSource->AddErrorToCurrentNode("errUnquotedAttributeStartLt");
-      return;
-    case '`':
-      mViewSource->AddErrorToCurrentNode("errUnquotedAttributeStartGrave");
-      return;
+  if (NS_LIKELY(mViewSource)) {
+    switch (c) {
+      case '=':
+        mViewSource->AddErrorToCurrentNode("errUnquotedAttributeStartEquals");
+        return;
+      case '<':
+        mViewSource->AddErrorToCurrentNode("errUnquotedAttributeStartLt");
+        return;
+      case '`':
+        mViewSource->AddErrorToCurrentNode("errUnquotedAttributeStartGrave");
+        return;
+    }
   }
 }
 
 void
 nsHtml5Tokenizer::errBadCharBeforeAttributeNameOrNull(PRUnichar c)
 {
-  if (c == '<') {
-    mViewSource->AddErrorToCurrentNode("errBadCharBeforeAttributeNameLt");
-  } else if (c == '=') {
-    errEqualsSignBeforeAttributeName();
-  } else if (c != 0xFFFD) {
-    errQuoteBeforeAttributeName(c);
+  if (NS_LIKELY(mViewSource)) {
+    if (c == '<') {
+      mViewSource->AddErrorToCurrentNode("errBadCharBeforeAttributeNameLt");
+    } else if (c == '=') {
+      errEqualsSignBeforeAttributeName();
+    } else if (c != 0xFFFD) {
+      errQuoteBeforeAttributeName(c);
+    }
   }
 }
 
 void
 nsHtml5Tokenizer::errBadCharAfterLt(PRUnichar c)
 {
-  mViewSource->AddErrorToCurrentNode("errBadCharAfterLt");
+  if (NS_LIKELY(mViewSource)) {
+    mViewSource->AddErrorToCurrentNode("errBadCharAfterLt");
+  }
 }
 
 void
 nsHtml5Tokenizer::errQuoteOrLtInAttributeNameOrNull(PRUnichar c)
 {
-  if (c == '<') {
-    mViewSource->AddErrorToCurrentNode("errLtInAttributeName");
-  } else if (c != 0xFFFD) {
-    mViewSource->AddErrorToCurrentNode("errQuoteInAttributeName");
+  if (NS_LIKELY(mViewSource)) {
+    if (c == '<') {
+      mViewSource->AddErrorToCurrentNode("errLtInAttributeName");
+    } else if (c != 0xFFFD) {
+      mViewSource->AddErrorToCurrentNode("errQuoteInAttributeName");
+    }
   }
 }
 
@@ -177,13 +189,17 @@ nsHtml5Tokenizer::errNcrControlChar(PRUnichar ch)
 void
 nsHtml5Tokenizer::errGarbageAfterLtSlash()
 {
-  mViewSource->AddErrorToCurrentNode("errGarbageAfterLtSlash");
+  if (NS_LIKELY(mViewSource)) {
+    mViewSource->AddErrorToCurrentNode("errGarbageAfterLtSlash");
+  }
 }
 
 void
 nsHtml5Tokenizer::errLtSlashGt()
 {
-  mViewSource->AddErrorToCurrentNode("errLtSlashGt");
+  if (NS_LIKELY(mViewSource)) {
+    mViewSource->AddErrorToCurrentNode("errLtSlashGt");
+  }
 }
 
 void
@@ -205,19 +221,25 @@ nsHtml5Tokenizer::errNoDigitsInNCR()
 void
 nsHtml5Tokenizer::errGtInSystemId()
 {
-  mViewSource->AddErrorToCurrentNode("errGtInSystemId");
+  if (NS_LIKELY(mViewSource)) {
+    mViewSource->AddErrorToCurrentNode("errGtInSystemId");
+  }
 }
 
 void
 nsHtml5Tokenizer::errGtInPublicId()
 {
-  mViewSource->AddErrorToCurrentNode("errGtInPublicId");
+  if (NS_LIKELY(mViewSource)) {
+    mViewSource->AddErrorToCurrentNode("errGtInPublicId");
+  }
 }
 
 void
 nsHtml5Tokenizer::errNamelessDoctype()
 {
-  mViewSource->AddErrorToCurrentNode("errNamelessDoctype");
+  if (NS_LIKELY(mViewSource)) {
+    mViewSource->AddErrorToCurrentNode("errNamelessDoctype");
+  }
 }
 
 void
@@ -231,7 +253,9 @@ nsHtml5Tokenizer::errConsecutiveHyphens()
 void
 nsHtml5Tokenizer::errPrematureEndOfComment()
 {
-  mViewSource->AddErrorToCurrentNode("errPrematureEndOfComment");
+  if (NS_LIKELY(mViewSource)) {
+    mViewSource->AddErrorToCurrentNode("errPrematureEndOfComment");
+  }
 }
 
 void
@@ -245,37 +269,49 @@ nsHtml5Tokenizer::errBogusComment()
 void
 nsHtml5Tokenizer::errSlashNotFollowedByGt()
 {
-  mViewSource->AddErrorToCurrentSlash("errSlashNotFollowedByGt");
+  if (NS_LIKELY(mViewSource)) {
+    mViewSource->AddErrorToCurrentSlash("errSlashNotFollowedByGt");
+  }
 }
 
 void
 nsHtml5Tokenizer::errNoSpaceBetweenAttributes()
 {
-  mViewSource->AddErrorToCurrentNode("errNoSpaceBetweenAttributes");
+  if (NS_LIKELY(mViewSource)) {
+    mViewSource->AddErrorToCurrentNode("errNoSpaceBetweenAttributes");
+  }
 }
 
 void
 nsHtml5Tokenizer::errAttributeValueMissing()
 {
-  mViewSource->AddErrorToCurrentNode("errAttributeValueMissing");
+  if (NS_LIKELY(mViewSource)) {
+    mViewSource->AddErrorToCurrentNode("errAttributeValueMissing");
+  }
 }
 
 void
 nsHtml5Tokenizer::errEqualsSignBeforeAttributeName()
 {
-  mViewSource->AddErrorToCurrentNode("errEqualsSignBeforeAttributeName");
+  if (NS_LIKELY(mViewSource)) {
+    mViewSource->AddErrorToCurrentNode("errEqualsSignBeforeAttributeName");
+  }
 }
 
 void
 nsHtml5Tokenizer::errLtGt()
 {
-  mViewSource->AddErrorToCurrentNode("errLtGt");
+  if (NS_LIKELY(mViewSource)) {
+    mViewSource->AddErrorToCurrentNode("errLtGt");
+  }
 }
 
 void
 nsHtml5Tokenizer::errProcessingInstruction()
 {
-  mViewSource->AddErrorToCurrentNode("errProcessingInstruction");
+  if (NS_LIKELY(mViewSource)) {
+    mViewSource->AddErrorToCurrentNode("errProcessingInstruction");
+  }
 }
 
 void
@@ -305,13 +341,17 @@ nsHtml5Tokenizer::errNoNamedCharacterMatch()
 void
 nsHtml5Tokenizer::errQuoteBeforeAttributeName(PRUnichar c)
 {
-  mViewSource->AddErrorToCurrentNode("errQuoteBeforeAttributeName");
+  if (NS_LIKELY(mViewSource)) {
+    mViewSource->AddErrorToCurrentNode("errQuoteBeforeAttributeName");
+  }
 }
 
 void
 nsHtml5Tokenizer::errExpectedPublicId()
 {
-  mViewSource->AddErrorToCurrentNode("errExpectedPublicId");
+  if (NS_LIKELY(mViewSource)) {
+    mViewSource->AddErrorToCurrentNode("errExpectedPublicId");
+  }
 }
 
 void
@@ -453,19 +493,25 @@ nsHtml5Tokenizer::errEofInSystemId()
 void
 nsHtml5Tokenizer::errExpectedSystemId()
 {
-  mViewSource->AddErrorToCurrentNode("errExpectedSystemId");
+  if (NS_LIKELY(mViewSource)) {
+    mViewSource->AddErrorToCurrentNode("errExpectedSystemId");
+  }
 }
 
 void
 nsHtml5Tokenizer::errMissingSpaceBeforeDoctypeName()
 {
-  mViewSource->AddErrorToCurrentNode("errMissingSpaceBeforeDoctypeName");
+  if (NS_LIKELY(mViewSource)) {
+    mViewSource->AddErrorToCurrentNode("errMissingSpaceBeforeDoctypeName");
+  }
 }
 
 void
 nsHtml5Tokenizer::errHyphenHyphenBang()
 {
-  mViewSource->AddErrorToCurrentNode("errHyphenHyphenBang");
+  if (NS_LIKELY(mViewSource)) {
+    mViewSource->AddErrorToCurrentNode("errHyphenHyphenBang");
+  }
 }
 
 void
@@ -487,17 +533,23 @@ nsHtml5Tokenizer::errNcrZero()
 void
 nsHtml5Tokenizer::errNoSpaceBetweenDoctypeSystemKeywordAndQuote()
 {
-  mViewSource->AddErrorToCurrentNode("errNoSpaceBetweenDoctypeSystemKeywordAndQuote");
+  if (NS_LIKELY(mViewSource)) {
+    mViewSource->AddErrorToCurrentNode("errNoSpaceBetweenDoctypeSystemKeywordAndQuote");
+  }
 }
 
 void
 nsHtml5Tokenizer::errNoSpaceBetweenPublicAndSystemIds()
 {
-  mViewSource->AddErrorToCurrentNode("errNoSpaceBetweenPublicAndSystemIds");
+  if (NS_LIKELY(mViewSource)) {
+    mViewSource->AddErrorToCurrentNode("errNoSpaceBetweenPublicAndSystemIds");
+  }
 }
 
 void
 nsHtml5Tokenizer::errNoSpaceBetweenDoctypePublicKeywordAndQuote()
 {
-  mViewSource->AddErrorToCurrentNode("errNoSpaceBetweenDoctypePublicKeywordAndQuote");
+  if (NS_LIKELY(mViewSource)) {
+    mViewSource->AddErrorToCurrentNode("errNoSpaceBetweenDoctypePublicKeywordAndQuote");
+  }
 }
diff --git a/parser/html/nsHtml5TokenizerLoopPolicies.h b/parser/html/nsHtml5TokenizerLoopPolicies.h
index b098029216f2..70fae2fa396b 100644
--- a/parser/html/nsHtml5TokenizerLoopPolicies.h
+++ b/parser/html/nsHtml5TokenizerLoopPolicies.h
@@ -38,9 +38,13 @@
 #ifndef nsHtml5TokenizerLoopPolicies_h_
 #define nsHtml5TokenizerLoopPolicies_h_
 
+/**
+ * This policy does not report tokenizer transitions anywhere. To be used
+ * when _not_ viewing source.
+ */
 struct nsHtml5SilentPolicy
 {
-  static const bool viewingSource = false;
+  static const bool reportErrors = false;
   static PRInt32 transition(nsHtml5Highlighter* aHighlighter,
                             PRInt32 aState,
                             bool aReconsume,
@@ -51,9 +55,13 @@ struct nsHtml5SilentPolicy
   }
 };
 
+/**
+ * This policy reports the tokenizer transitions to a highlighter. To be used
+ * when viewing source.
+ */
 struct nsHtml5ViewSourcePolicy
 {
-  static const bool viewingSource = true;
+  static const bool reportErrors = true;
   static PRInt32 transition(nsHtml5Highlighter* aHighlighter,
                             PRInt32 aState,
                             bool aReconsume,
diff --git a/parser/html/nsHtml5TreeBuilderHSupplement.h b/parser/html/nsHtml5TreeBuilderHSupplement.h
index c0b5501aa588..0614e7c61b4f 100644
--- a/parser/html/nsHtml5TreeBuilderHSupplement.h
+++ b/parser/html/nsHtml5TreeBuilderHSupplement.h
@@ -58,6 +58,29 @@
      */
     void documentMode(nsHtml5DocumentMode m);
 
+    /**
+     * Using nsIContent** instead of nsIContent* is the parser deals with DOM
+     * nodes in a way that works off the main thread. Non-main-thread code
+     * can't refcount or otherwise touch nsIContent objects in any way.
+     * Yet, the off-the-main-thread code needs to have a way to hold onto a
+     * particular node and repeatedly operate on the same node.
+     *
+     * The way this works is that the off-the-main-thread code has an
+     * nsIContent** for each DOM node and a given nsIContent** is only ever
+     * actually dereferenced into an actual nsIContent* on the main thread.
+     * When the off-the-main-thread code requests a new node, it gets an
+     * nsIContent** immediately and a tree op is enqueued for later allocating
+     * an actual nsIContent object and writing a pointer to it into the memory
+     * location pointed to by the nsIContent**.
+     *
+     * Since tree ops are in a queue, the node creating tree op will always
+     * run before tree ops that try to further operate on the node that the
+     * nsIContent** is a handle to.
+     *
+     * On-the-main-thread parts of the parser use the same mechanism in order
+     * to avoid having to have duplicate code paths for on-the-main-thread and
+     * off-the-main-thread tree builder instances.)
+     */
     nsIContent** AllocateContentHandle();
     
     void accumulateCharactersForced(const PRUnichar* aBuf, PRInt32 aStart, PRInt32 aLength)
diff --git a/parser/html/nsHtml5TreeOpExecutor.cpp b/parser/html/nsHtml5TreeOpExecutor.cpp
index df3c0447473a..b85e960deab2 100644
--- a/parser/html/nsHtml5TreeOpExecutor.cpp
+++ b/parser/html/nsHtml5TreeOpExecutor.cpp
@@ -905,6 +905,8 @@ nsHtml5TreeOpExecutor::GetViewSourceBaseURI()
       NS_ASSERTION(nested, "URI with scheme view-source didn't QI to nested!");
       nested->GetInnerURI(getter_AddRefs(mViewSourceBaseURI));
     } else {
+      // Fail gracefully if the base URL isn't a view-source: URL.
+      // Not sure if this can ever happen.
       mViewSourceBaseURI = orig;
     }
   }
diff --git a/parser/html/nsHtml5TreeOperation.cpp b/parser/html/nsHtml5TreeOperation.cpp
index 8507159a1d96..bfae5c444a6b 100644
--- a/parser/html/nsHtml5TreeOperation.cpp
+++ b/parser/html/nsHtml5TreeOperation.cpp
@@ -715,6 +715,7 @@ nsHtml5TreeOperation::Perform(nsHtml5TreeOpExecutor* aBuilder,
       nsIContent* node = *(mOne.node);
       PRUnichar* str = mTwo.unicharPtr;
       nsDependentString depStr(str);
+      // See viewsource.css for the possible classes
       nsAutoString klass;
       node->GetAttr(kNameSpaceID_None, nsGkAtoms::_class, klass);
       if (!klass.IsEmpty()) {
@@ -760,8 +761,7 @@ nsHtml5TreeOperation::Perform(nsHtml5TreeOpExecutor* aBuilder,
       rv = NS_URIChainHasFlags(uri,
                                nsIProtocolHandler::URI_OPENING_EXECUTES_SCRIPT,
                                &openingExecutesScript);
-      NS_ENSURE_SUCCESS(rv, NS_OK);
-      if (openingExecutesScript) {
+      if (NS_FAILED(rv) || openingExecutesScript) {
         return NS_OK;
       }
 
@@ -795,6 +795,7 @@ nsHtml5TreeOperation::Perform(nsHtml5TreeOpExecutor* aBuilder,
       char* msgId = mTwo.charPtr;
       nsCOMPtr<nsIAtom> atom = Reget(mThree.atom);
       nsCOMPtr<nsIAtom> otherAtom = Reget(mFour.atom);
+      // See viewsource.css for the possible classes in addition to "error".
       nsAutoString klass;
       node->GetAttr(kNameSpaceID_None, nsGkAtoms::_class, klass);
       if (!klass.IsEmpty()) {