no system id specified

Cause:

Your document includes a DOCTYPE declaration with a public identifier (e.g. "-//W3C//DTD XHTML 1.0 Strict//EN") but no system identifier (e.g. "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"). This is authorized in HTML (based on SGML), but not in XML-based languages.

Example:

textexample textexample

Bad:
textexample textexample
Good:
textexample textexample

Solution:

textexample textexample

References: