Firefox dev edition has been my go to lately, highly recommended if you do a lot of markup / css debugging. Being able to see outlines around elements as I hover / scan through the DOM easily doubles my speed.
I have found the CSS "changes" tab to be very useful. Just tweak the CSS and once you're satisfied you can just view a diff and use that to modify your code base [0]
[0]: https://developer.mozilla.org/en-US/docs/Tools/Page_Inspecto...