Validating an XHTML document's content involves checking its markup against a DTD and reporting markup errors. You can validate HTML, XHTML, and CSS files by using the W3C's validator. It will allow you to validate files on the Internet or on your computer.
The validator program will go through your coding, find any errors, and create a report. You can then "clean up" your coding and validate again. This service is free.
You can also check your external CSS pages through the CSS validator ...
Once validated as well formed coding, the bottom of the report page will show an icon image that you may display on your page such as the ones you see in the right side column of my site. Display it with pride!