Building a cross-browser website is a pain in the ass, but there's someone on the the net who can help you with this job: http://www.quirksmode.org/
I fouded Peter-Paul Koch's site because I had problems developing a cross-browser javascript code and I found it really interesting and easy to understand. After a quite deep reading I can confirm what Peter-Paul says: QuirksMode.org is the prime source for browser compatibility information on the Internet.

His best project can be found in the Compatibility Master Table where you can check before your development, if your ideas will work fine on almost every browser. Probably he is right when he says: "His DOM Compatibility Tables are reliably estimated to have saved the global web developer population about € 5 million on hair transplants in 2007 alone."

I wish i founded this site earlier...