What is the problem

Validation and typing are separable but often conflated concepts.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [
<!ATTLIST foo partnum ID #IMPLIED> ]>
<foo  partnum="i54321" bar="toto"/>

• The instance is well formed
• The instance is not valid(atable)
• The partnum attribute on foo is of type ID