I have Safari 3/OSX, Safari 2/OSX, Firefox 2/OSX, Firefox 3/OSX, Opera 9/OSX, Internet Explorer 7/XP (on Parallels), Internet Explorer 6 Standalone/XP (on Parallels), and Firefox 2/XP (on Parallels) installed on my Macbook.
That’s not enough because apparently Internet Explorer 7/Vista and Internet Explorer 6/XP (not Standalone and not on Parallels) behave differently still. I guess I need to install Vista on my Macbook too. Awesome.