Safari Enhancer enables the debug menu (and a couple of other things) in Safari, Apple's new web browser. The debug menu lets you browser the DOM and, nicely, provides an interface for testing how arbitrary html snippets are rendered.

If you don't want to use the enhancer application, go to the Terminal and type "defaults write IncludeDebugMenu 1" to switch the debug menu on, and "defaults delete IncludeDebugMenu" to remove it.