During the stage of css/html testing you have to test with multiple web browsers and operating systems.
Unfortunately in Mac OS X its a little difficult to have multiple Safari versions, since the Webkit framework is what is updated and webkit is system-wide, you can’t have both at the same time…
Until I discover this clever man, who embed the webkit in an app, so you can have your own safari 3 and safari 4 running on your mac… so you can test everything … go and download it, you will never regret it: