Andy, for you!

Simon Willison: Using bookmarklets to experiment with CSS

bq. by far the most useful for learning CSS are test styles and edit styles. test styles opens a popup window in which you can type CSS rules that will be applied instantly to the page you are viewing, while edit styles does the same thing but pre-populates the window with the style sheets of the current site, allowing you to edit them in place. ancestors is also useful; it shows the element hierarchy of the element your mouse is currently over, which can help you see what selector you need to use to modify that element’s style. I cannot emphasize enough how powerful this is for working with CSS; using test styles I was able to redesign this site entirely in less than half an hour!