If I understand correctly, you want to be notified that the code you've written in the Template is invalid HTML.
If so, I'm afraid there is no easy way. The Template is purely producing text that you specify to go out to the response.
It may not even be HTML - could be JavaScript or a number of other outputs - it's just a string of text.
You may have a Template that produces the start of a table, another that produces the body, and another that produces the footer and end table tags. Each of these would produce invalid HTML on their own, but output one after the other would produce a valid HTML table. (That's a lot of produces there - sorry).
What would make it invalid is the parser of the HTML - i.e. the browser. You would only be able to validate your Template output when it is in a complete document that can then be parsed.