I don't know if it needs direct access to dom diff. But if it recursively invokes the serializer for a subtree, it should be a SelectiveSerializer; ie we should be doing selser and domdiff on that subtree. And we should be smart enough that if a subtree of an extension tag mismatches, we should pop up and reserialize the entire extension tag (using the extension tag's handler), not try to splice it in ourselves.