So, there is of course the great W3C Markup Validation Service for checking the HTML of your pages. I recently came across Checky, a very handy plug-in for Mozilla that lets you instantly access a whole host of different validators and checkers from a contextual menu. And if that’s too much work, just pressing F10 can open a new tab with the current page ruthlessly judged in your validator of choice. I’ve only tried it on Windows, but it makes life just that little bit easier.
Also, I recently, and belatedly, came across Amaya, the web browser the W3C use “to demonstrate and test many of the new developments in Web protocols and formats.” Yes, it has all sorts of Berners-Lee type things like a built-in page editor and collaboration tools. But for the purposes of this post, it also provides some kind of validation — open a page and if there are “Parsing Errors” it tells you what’s up. Well, it does for the XHTML pages I’ve tried anyway. Clicking from page to page and having each tested in some way is nice and easy. Not a substitute for the proper validator, but handy nonetheless.