Use the W3C Markup Validation Service to validate your code. Go to http://validator.w3.org/ to run the validator. It will even give you hints on how to fix it. There are three ways you can validate your code:

1. URI
2. File Upload
3. Direct Input

After your code has been validated add the Official W3C Validation Logo to your site to let people know that your site has been validated.

<p>
    <a href="http://validator.w3.org/check?uri=referer">
        <img src="http://www.w3.org/Icons/valid-xhtml10"
             alt="Valid XHTML 1.0 Strict" height="31" width="88" /></a>
</p> 
        

Just copy and paste the code into your site once it passes the validation test. Visit the site for a selection of more icons and the corresponding snippet of code.

This page was validated and passed!