There are times when browsers render CSS and HTML differently depending on the platform and browser being used. Usually, I can check for cross-browser consistency using only Windows and variety of installed browsers for Windows (Internet Explorer, FireFox, Safari, Google Chrome, etc.). However, there is sometimes major differences in how the PC version of a browser renders a page v/s the Mac version of a particular browser. I have noticed this most in Safari between Windows and Mac.

I do not own a Mac and cannot test directly on that platform, so I have had to rely on online services to show me how a page gets rendered in Safari on a Mac.

Here is a great online service that is FREE and produces a high quality JPG image of how the page will look on Mac in Safari.
BrowserCamp - http://www.browsrcamp.com/

For other platforms (Linux, Windows, Mac) and other browser types (Internet Explorer, FireFox, Opera, & others), this online service can be used as well to test a site's display. This site also provides options for testing in older browser versions as well.
BrowserShots - http://browsershots.org/