diff --git a/mozilla/htmlparser/src/nsExpatTokenizer.cpp b/mozilla/htmlparser/src/nsExpatTokenizer.cpp
index 897bdee6262..66154226fe6 100644
--- a/mozilla/htmlparser/src/nsExpatTokenizer.cpp
+++ b/mozilla/htmlparser/src/nsExpatTokenizer.cpp
@@ -364,10 +364,11 @@ nsExpatTokenizer::AddErrorMessageTokens(nsParserError* aError)
  * in the content sink:
  *
  *    <ParserError>
- *       XML Error: "contents of aError->description"
- *       Line Number: "contents of aError->lineNumber"
+ *       XML Error: [aError->description]
+ *       Location: [aError->sourceURL]
+ *       Line Number: [aError->lineNumber], Column [aError->colNumber]
  *       <SourceText>
- *          "Contents of aError->sourceLine"
+ *          [aError->sourceLine]
  *          "^ pointing at the error location"
  *       </SourceText>
  *    </ParserError>
@@ -387,6 +388,58 @@ nsExpatTokenizer::PushXMLErrorTokens(const char *aBuffer, PRUint32 aLength, PRBo
     // Adjust the column number so that it is one based rather than zero based.
     error->colNumber = XML_GetCurrentColumnNumber(mExpatParser) + 1;
     error->description.AssignWithConversion(XML_ErrorString(error->code));
+    if (error->code==XML_ERROR_TAG_MISMATCH){
+      /*
+       * Certain things can be assumed about the token stream because of
+       * the way expat behaves.  eg:
+       *
+       *  - data:text/xml,</foo> is NOT WELL-FORMED
+       *  - data:text/xml,<foo></bar> is a TAG_MISMATCH.
+       *
+       * We can assume that there is at least one extra open tag (the one we 
+       * want), so balance is initially set to one.
+       *
+       * Then loop through the tokens:
+       *  - Each time we see eToken_end (</tag>) increment balance, because 
+       *    that means there is another pair of tags we don't care about.
+       *  - Each time we see eToken_start (<tag>) decrement balance because it
+       *    matches a close tag (perhaps the MISMATCHed tag in which case 
+       *    balance should hit 0).
+       *  - If balance ever hits zero, exit the loop.  Because of the way  
+       *    balance is adjusted, if balance is zero expected *must* be a start
+       *    tag.
+       *
+       * We must check expected in the condition in case expat or nsDeque go
+       * crazy and give us 0 (null) before balance reaches 0.
+       */
+      nsDequeIterator current = mState->tokenDeque->End();
+      CToken *expected = NS_STATIC_CAST(CToken*,--current);
+      PRUint32 balance = 1;
+
+      while (expected) {
+        switch (expected->GetTokenType()) {
+          case eToken_start:
+            --balance;
+            break;
+          case eToken_end:
+            ++balance;
+            break;
+          default:
+            break; // we don't care about newlines or other tokens
+        }
+
+        if (!balance) {
+          // if balance is zero, this must be a start tag
+          CStartToken *startToken=NS_STATIC_CAST(CStartToken*,expected);
+          error->description.Append(NS_LITERAL_STRING(". Expected: </"));
+          error->description.Append(startToken->GetStringValue());
+          error->description.Append(NS_LITERAL_STRING(">"));
+          break;
+        }
+
+        expected = NS_STATIC_CAST(CToken*,--current);
+      }
+    }
     error->sourceURL.Assign((PRUnichar*)XML_GetBase(mExpatParser));
     if (!aIsFinal) {
       PRInt32 byteIndexRelativeToFile = 0;
diff --git a/mozilla/parser/htmlparser/src/nsExpatTokenizer.cpp b/mozilla/parser/htmlparser/src/nsExpatTokenizer.cpp
index 897bdee6262..66154226fe6 100644
--- a/mozilla/parser/htmlparser/src/nsExpatTokenizer.cpp
+++ b/mozilla/parser/htmlparser/src/nsExpatTokenizer.cpp
@@ -364,10 +364,11 @@ nsExpatTokenizer::AddErrorMessageTokens(nsParserError* aError)
  * in the content sink:
  *
  *    <ParserError>
- *       XML Error: "contents of aError->description"
- *       Line Number: "contents of aError->lineNumber"
+ *       XML Error: [aError->description]
+ *       Location: [aError->sourceURL]
+ *       Line Number: [aError->lineNumber], Column [aError->colNumber]
  *       <SourceText>
- *          "Contents of aError->sourceLine"
+ *          [aError->sourceLine]
  *          "^ pointing at the error location"
  *       </SourceText>
  *    </ParserError>
@@ -387,6 +388,58 @@ nsExpatTokenizer::PushXMLErrorTokens(const char *aBuffer, PRUint32 aLength, PRBo
     // Adjust the column number so that it is one based rather than zero based.
     error->colNumber = XML_GetCurrentColumnNumber(mExpatParser) + 1;
     error->description.AssignWithConversion(XML_ErrorString(error->code));
+    if (error->code==XML_ERROR_TAG_MISMATCH){
+      /*
+       * Certain things can be assumed about the token stream because of
+       * the way expat behaves.  eg:
+       *
+       *  - data:text/xml,</foo> is NOT WELL-FORMED
+       *  - data:text/xml,<foo></bar> is a TAG_MISMATCH.
+       *
+       * We can assume that there is at least one extra open tag (the one we 
+       * want), so balance is initially set to one.
+       *
+       * Then loop through the tokens:
+       *  - Each time we see eToken_end (</tag>) increment balance, because 
+       *    that means there is another pair of tags we don't care about.
+       *  - Each time we see eToken_start (<tag>) decrement balance because it
+       *    matches a close tag (perhaps the MISMATCHed tag in which case 
+       *    balance should hit 0).
+       *  - If balance ever hits zero, exit the loop.  Because of the way  
+       *    balance is adjusted, if balance is zero expected *must* be a start
+       *    tag.
+       *
+       * We must check expected in the condition in case expat or nsDeque go
+       * crazy and give us 0 (null) before balance reaches 0.
+       */
+      nsDequeIterator current = mState->tokenDeque->End();
+      CToken *expected = NS_STATIC_CAST(CToken*,--current);
+      PRUint32 balance = 1;
+
+      while (expected) {
+        switch (expected->GetTokenType()) {
+          case eToken_start:
+            --balance;
+            break;
+          case eToken_end:
+            ++balance;
+            break;
+          default:
+            break; // we don't care about newlines or other tokens
+        }
+
+        if (!balance) {
+          // if balance is zero, this must be a start tag
+          CStartToken *startToken=NS_STATIC_CAST(CStartToken*,expected);
+          error->description.Append(NS_LITERAL_STRING(". Expected: </"));
+          error->description.Append(startToken->GetStringValue());
+          error->description.Append(NS_LITERAL_STRING(">"));
+          break;
+        }
+
+        expected = NS_STATIC_CAST(CToken*,--current);
+      }
+    }
     error->sourceURL.Assign((PRUnichar*)XML_GetBase(mExpatParser));
     if (!aIsFinal) {
       PRInt32 byteIndexRelativeToFile = 0;