XML is only concerned with whether a document is well-formed, not its conformity to a given schema. Schemas like XSD, DTD, etc can be plugged in later. Many systems just have an ad hoc schema.

> At least with JSON and HTML, you don't need a separate definition file for basic, primitive data types.

Unless I’m missing your meaning, this seems like an apples-to-oranges comparison. HTML is not a general-purpose format like JSON. It’s a very complicated document format that is validated with reference to an external spec.

I think XML is a great fit for a document format that can become arbitrarily complex yet still easy to author and validate. It’s obviously a really poor fit for a wire transport protocol.