XHTML Validator 1.0
XHTML Validator can validate XHTML documents. It may also validate other types of XML documents. It can validate a XHTML document first determining whether it is well-formed and then verifying whether it uses only valid tags and attributes. The list of tags and attributes accepted as valid is configurable.
If there is an error, it can provide detailed information about what ist wrong. It uses PHP 5 syntax and exceptions for handling errors.