Why?
The document is invalid XHTML or uses elements or structures that are not allowed in the XHTML Basic 1.1 specification. This is likely to be problematic for mobile browsers who are more sensitive to the structure of the content than desktop browsers.
How?
Check the page with the W3C Markup Validator
While unknown elements are likely to be ignored by browsers, a simple syntax error may invalidate the document to the point where browsers cannot render it anymore. Also bear in mind that checks were run against a tidied version of the document. The tidying process may have modified the structure and semantics of the original document and may have either created new problems or fixed existing ones.
Where?
Triggered 4 times by the resource under test: