Via Paul, check out these excellent topographic page layout bookmarklets. They show the "height" of the elements on a web page by shading the more deeply-nested elements with a lighter background color. It's much better in some ways than border-based bookmarklets that do similar things. It's easier on the eyes, and it preserves the page layout exactly (adding borders changes the size of elements, and therefore changes the layout).
Try it on my site! (refresh to get things back to normal)
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