Russell Beattie NotebookRussell Beattie points to a very nice Mozilla/Firefox extension...

It allows you to edit your web page's CSS in the left sidebar and watch the change in realtime in the browser pane.

I installed this on Firefox and it works very well. Just call it from the Sidebar submenu and the CSS of the page that is currrently in the browser window shows up in a pane to the left. Start to edit the CSS and see the changes instantly. This is great for experimenting on the fly.