Firebug
Firebug an add-on to FireFox is a perfect tool for web developers/designers it shows any browsed page HTML, CSS, DOM etc... and you can modify and see the results instantly, deactivate or modify the CSS code and see instantly what will the page looks like.
Any way you got to try it if you are using FireFox if you are not you have to, if you insists you may use Firebug Lite.