To make a local style sheet override the site's own, the rules in question need to be flagged as !important, as in
Code:
div#subcontent, div#outerslice, div#bloginfo, div#grid { display: none !important }
Put this in a CSS file and hide the ads on engadget.com!
The style sheet can also be named whatever you want, as long as it's selected in the site preferences window.
I use this all the time to hide ads and other annoyances. Actually, OW's implementation is the best. you can make changes to the css file and just "reload" the page to see the results. In FF or Camino, you have to quit the app & re-launch.