If you do not go with DTD or XSD, you are only doing XML lookalike language, as these are XML mechanisms to really define the XML schema: a compliant parser won't be able to validate it, or maybe even to parse it.

Thus people go with custom parsers (how hard can it be, right?), and then have to keep fixing issues as someone or other submits an XML with CDATA in or similar.

What if we just formalize some reasonable minimal subset, and call it something else?