Yes, though many languages have lenient parsers. Most browser parsers, for example, will probably only be lenient if parsing "HTML."

    new XMLSerializer().serializeToString(new DOMParser().parseFromString("<a>hello < </a>", "text/html")) 

The above in my console does as expected there. And again, entities are a very dangerous part of XML and friends.

You are correct that if you tell it that that is xml, the browser will throw it back at you. Just as the JSON parser will barf on JSON.parse("{'test':'value'}").