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The best way to make sure you have valid cross browser code is to write to the validator. Just make sure everything on the page validates.
I already know that some osX users have complained to me about some javascript function not working. (THEY didn't know it was javascript and thankfully they called me.) I fixed it but I expect more errors to come.
Writing valid code is certainly sound advice, but we all know that each browser has a few valid things that it can't render, and we learn to avoid those few things where possible. Because Safari is the newest I hope that it is the best at rendering valid code correctly, but no browser is ever perfect and Safari won't be an exception. So if anyone discovers a rendering issue in Safari, please post it here!