No system id specified

Cause:

Your document includes a DTD for that language). Omitting the system identifier is authorized in HTML (based on SGML), but not in XML-based languages.

If you are using a standard XHTML document type, it is recommended to use exactly one of the DOCTYPE declarations from the recommended list on the W3C QA Website.

Example:

Good
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN">
<html lang="en">
...
Good
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html lang="en">
...

Solution:

Add the URL pointing to the DTD in the DOCTYPE

References: