Some of you may have known about this really cool extension, but I just discovered it and have to share my joy!
This neat extension adds a wee button at the bottom of your screen, and each site you go to gets validated. If the site works, you get a white tick in a green circle, if there are warnings, you get an exclamation mark in a yellow triangle, and if it there are errors, you get a big x and a red circle. Hovering over it gives you bit more information and you can get full details too.
It's an interesting tool to see at a glance what works and what doesn't.
And the scary bit is, not all that many of the sites I go to on a regular basis actually get the green "all clear", and too many get a red "error" sign.