In Designing with Web Standards, Jeffrey Zeldman advocates designing for the best browser you have on hand and then testing in the lesser ones, contrary to old-school methods of web design.
The theory, I believe, is that you are then building your site in closest possible conformance to the official W3C specs.
Until recently for example, I’d build sites in IE6/Win, and then ‘hack’ them to work in Mozilla. This only lead to problems down the line, since the original rendering relied on a buggy rendering engine (IE6) and created dependencies on it that better browsers won’t handle the way you’d expect them to. I noticed more than once that if I had to hack my CSS to make it work in Mozilla, Mac browsers would do very unpredictable things to the resulting code.
So now I develop in Firebird, and test later in IE. Is it better? I can’t say for sure yet. I’ve developed one site this way: this one. But I’ve never seen it in Safari, and no one using Safari has reported errors. That has to count for something. What has your experience been with this method of testing?