Modi v2
If you haven't heard of Modi and you're a web page editor where have you been???
Check it out at the Modi v2 homepage. It's a fantastic bookmarklet from Slayer Office (gotta love their tag line, "Web experiments gone horribly awry" ;-).
To use it you just click the bookmark of the Modi v2 link and up will pop a translucent window that tells you the properties of whatever you hover over in the page, in great detail. Plus it comes with keyboard commands to do all kinds of wacky and useful things.