Stop the press! Found an amaz­ingly use­ful exten­sion, thanks to Andy Jar­rett — a HTML val­ida­tor based on Tidy that runs in your Fire­fox sta­tus bar, giv­ing you a count of errors and warn­ings on the cur­rent page.

Now, even if I don’t bother to val­i­date my pages, that lit­tle sta­tus bar will bug me any­where it doesn’t show me a lit­tle green tick. Damn that’s use­ful! I got all the devel­op­ers in the office to install it yes­ter­day, it will improve my pro­duc­tiv­ity a heap by automat­ing my nag­ging. I’m excited! If you double-click on the sta­tus bar score it opens up a source view win­dow with the errors and warn­ings high­lighted, and even has a lit­tle expla­na­tion win­dow down the bot­tom. It can also attempt to fix the prob­lems for you, and presents before-and-after html source and browser views.

The only caveat is how Tidy clas­si­fies errors and warn­ings — any­thing that it can cor­rect auto­mat­i­cally is a warn­ing, any­thing that com­pletely mys­ti­fies it is an error. So, a miss­ing DOCYTPE dec­la­ra­tion or an invalid attribute is just a warn­ing — Tidy says “hey, no prob­lem, I’ll just fix that right up for you” — whereas the W3C val­ida­tor would get all medieval on yo’ ass over the same issues. I guess it’s a case of not being happy with a page until there’s a lit­tle green tick in the bot­tom of your screen.

Cos it runs in your own browser, it works behind fire­walls, on pages requir­ing authen­ti­ca­tion, etc.

Check it out for your­selves — HTML Val­ida­tor for Fire­fox and Mozilla.