[mikemaccana]

You can already do this in Chrome without doing anything (in Google fashion, though, the UI isn't very intuitive).

Check Paul Irish's Chrome Dev Tools talk from JSConf 2011 for that and other tips.

[evilduck]

For the lazy: http://paulirish.com/2011/a-re-introduction-to-the-chrome-de...

(edit: around the 8:30 mark)

[primigenus]

Not when we made this, you couldn't ;-)