A central idea of XML was that - in reaction to the mess that was HTML - any tool that calls itself XML MUST barf loudly on anything that is not XML, so that you could never have a situation where one tool is happily calling something XML and another tool barfs on it. (Because, given the choice, humans will regularly mess it up but not notice unless their tool tells them so, and what works for one tool won't for another.)

The requirement of strictness everywhere is a precondition for interoperability between diverse toolsets.