Whenever you start work on a new theme you want to make sure that all the code validates properly. While in the long run you'll find yourself at the mercy of code from Google, YouTube and other widget producers you definitely want your code to be as clean as possible.

The W3C offers a number of tools for validating everything from HTML to CSS but to use them you must manually enter the URL of each page you want to validate.

HTML ValidatorThat's why I use the Firefox HTML Validator extension.

This extension gives you a quick snapshot at your code and instantly tells you if there are any errors that prevent you from having "valid" code. It resides in the lower portion of the Firefox interface and simply shows you an icon telling you if you're site validates or not.

Clicking on the icon will provide a window that allows you to quickly jump between errors on the page too - which makes debugging and cleaning things up much easier. If you're using Firefox grab this extension ASAP.