Thanks for your interest in LanguageTool! I don’t get that error, but a different one: & must be & in XML. Also, there needs to be a <marker>...</marker> element in the example. I’m not sure where the error with h3 comes from, but please make sure that you paste the rule only from a plain text editor, not from a text processing software.
LanguageTool - Supported Languages in this url in that table list, there is no tamil spell checker. My question is shall I use LT for tamil spell checker? I wrote a rules but I am not able to use it. Please help me