Well-formed XML

"But XML allows me to create my own language. Surely there are no rules". Well, yes and no.

While XML is not restricted by a set of available tags and it allows you to create your own markup, there are still overarching rules that you must follow. Adherence to these rules results in XML that is well‑formed. By definition, a well-formed XML document is one that syntactically conforms to the World Wide Web Consortium (W3C) XML Specification.

Essentially, using the spoken language parallel, this means that the right components of a sentence must be included and spelled correctly. An example in XML is that a processing instruction must always begin with the <? characters.