This is neat. Holmes is The CSS Markup Detective:
Holmes is stand-alone diagnostic CSS stylesheet that can highlight potentially invalid, inaccessible or erroneous HTML(5) markup by adding one class.
I like this because it’s in a way passive, but really useful. When I was really getting into Web Standards, I had a number of Firefox Add-ons that would light up, blink, and cause a bit of chaos when things were invalid or errors were present. I’ve outgrown that to an extent, but I like the idea of this. I’ll be keeping an eye on it, keeping yourself in check is still important.