If you are into web development you probably have to code some HTML from time to time if not constantly. And if you are anything like me you always try to get your HTML as valid as possible (I mean, come on - there are cases when you just say ‘screw it’). So while the W3 Markup Validation Service is nice and stuff, its not really what you call convenient.
So if you don’t know it already you should get HTML Validator which is a Firefox plugin. It validates all sites or just the ones you explicitly enabled it for. It also shows where there are mistakes in your HTML and will give you the same hints the W3 validator gives. That is because both use the OpenSP SGML parser.
All very convenient (I’m over-using this word a bit no?) and perfect for people that constantly are short on time or simply are lazy like me.