If you’re a web designer or even a blogger who likes to tinker with html, I’d strongly suggest getting this tiny little plugin for Firefox. I use it all the time to ensure all my sites adhere to the strictest web standards.

HTML Validator plugin (download)

It’s a clever extension that adds HTML validation inside Firefox and Mozilla that displays the number of errors on a page in the form of an icon in your status bar while browsing.

By viewing the HTML source of the page in your browser you can see not only details of the errors, but explanations on why and how to fix them. It can even automatically fix your page for you.

The extension is based on Tidy – originally developed by the Web Consortium W3C and now extended and improved by a lot of persons. Tidy is embedded inside Mozilla/Firefox and makes the validation locally on your machine, without sending HTML to a third party server.

It finds HTML errors and classifies them in 3 categories:

• errors: HTML errors that Tidy cannot fix or understand.
• warnings: HTML errors that Tidy can fix automatically
• (optional) accessibility warnings: HTML warnings for the 3 priority levels defined in W3c WAI

The extension is available on Windows, MacOsX and Linux.