Correct vocabulary: all elements and attributes actually exist in that
language
Correct grammar: elements are used in the right place, in the right order
In addition to well-formedness (conforming to the rules of XML as a metalanguage),
there is also validity which requires conforming to the rules of a specific language,
such as TEI or HTML.
These rules are very similar to the rules for speaking in a human language: am I using
the right words? and am I using them in the correct manner?
When we check an XML document for validity, we are making sure that the document obeys
the rules of a specific language.
