In my list of essen­tial web devel­op­ment exten­sions for Fire­fox, a really handy one got missed out: HTML Val­ida­tor based on Tidy.

I for­got about it because it sits, qui­etly, in the bot­tom right of the browser sta­tus bar. For every page you visit, it dis­plays the num­ber of HTML “errors” and “warn­ings” it finds. Double-click on the dis­play to view source with the items flagged, or right-click to get a list of options. If you want, Tidy can attempt to fix the prob­lems for you, and a lot of the time it does a pretty good job. Hav­ing it down the bot­tom is great because you don’t need to val­i­date the page, you can see right away if there’s any­thing wrong.

Now, the actual mean­ing of the terms “errors” and “warn­ings”, accord­ing to Tidy, is a lit­tle prob­lem­atic. “Warn­ings” refers to any­thing that Tidy thinks it can fix itself. “Errors” refers to things it doesn’t know what to do with. So, even the most dire val­i­da­tion errors can be marked as “warn­ings”. Which means you have to train your­self to use it… if there’s any­thing other than a green tick, there’s work to be done!

Despite that, HTML Val­ida­tor is a really handy extension.