Each XHTML document is validated against a Document Type Description. Until validating an XHTML file properly, a appropriate DTD must be inserted as the first or second line of the file.
When you are able to validate your XHTML document, you will use W3C Validator to verify the document. This tool is really useful and allows you to solve the problems with your document. It method does not need any experience to conduct validation.
The quoted statement in the text box displays you information. You need to give complete URL of the page, which you want to validate and then click Validate Page button.
This validator tests the markup validity of web documents with different formats particularly in HTML, XHTML, SMIL, MathML, etc.