[vog]

This is a great idea, although for productive use I'd prefer that as a browser extension or bookmarklet, rather than having to visit a foreign website (haxx.ly). On the other hand, that separate website is a comfortable way to show my results to others.

[vhf]

Stylebot (chrome extension) lets you do that. That's actually what I use for HN, by example.

It's a really good extension. Click it, click directly on your page to select a DOM element, enter any valid CSS, and it's saved for your next visit.