HTML Validator Firefox Add-On
First, let's look at why you need an HTML Validator in the first place. Ever wanted to see how many HTML “bugs” are on each page of your site at a glance? Now, while each page may be operating just fine, there are some issues with browser compatibility that have plagued developers ever since the days of Netscape versus Internet Explorer. Each company saw the standard HTML guidelines, developed a browser for it, and made their own neat additions to the development of HTML. That gave rise to many websites having some sort of statement along the lines of “This website is best viewed in” followed by either Netscape or Internet Explorer (or, in the case of a really die-hard nerd, Lynx). That's because the page could look very different and lose functionality if you use your preferred browser as opposed to theirs. Now with Safari, Firefox, Opera, Internet Explorer, and every other browser out there with multiple versions, operating systems, and plug-ins, your website can look like a mess to the majority of users if you don't follow the W3C guidelines. If you didn't know already, now you know why you need an HTML Validator.
Why You Should Use the Firefox HTML Validator Add-On
To validate that your code is compliant with the W3C guidelines, you could always go to validator.w3.org and enter in the URL for each page manually, upload the files, or validate a piece of code by copy pasting it into a dialog box. That can be a big pain, obviously, and distract you from your work by having to go through hoops as you upload, check, and fix in a loop.
The HTML Validator Add-On uses the same algorithm as the W3C's web-based validator, and it also uses the Tidy algorithm which was also developed by the W3C. So, with the free add-on installed, you can quickly access your pages locally, validate your pages locally, and it will pop up an icon showing you how many there are on that page. It does it automatically and quickly, so you cut out the time consuming busy work.
Download HTML Validator