XHTML - Validations
Every XHTML document is validated against a Document Type Definition. Before validating an XHTML file properly, a correct DTD must be added as the first or second line of the file.
Once you are ready to validate your XHTML document, you can use W3C Validator to validate your document. This tool is very handy and helps you to fix the problems with your document. This tool does not require any expertise to perform validation.
The following statement in the text box shows you details. You need to give complete URL of the page, which you want to validate and then click Validate Page button.
Input your page address in the box below:
This validator checks the markup validity of web documents with various formats especially in HTML, XHTML, SMIL, MathML, etc.
There are other tools to perform different other validations.