IE3 gave me a bit of a challenge because it likes cookies to only be set a certain way, JUST SO.
However, the most adjustment was probably for Mosaic, who treats ">", not "-->" as HTML comment closing, so my JavaScript can't have > characters. For if statements, I flip them, and anytime I need a > character, I get it from the char value. (Almost all my JS is baked into the HTML by the templating engine, because some browsers don't like/support external includes.)
I don't have an extensive Mac IE testing stable yet, but I have tested in IE5 Mac, which has its own rendering engine called Tasman.
However, with every browser I add support for it becomes more and more likely that another one I try will also work, especially if it came later. Netscape 2.0 works, so that covers a good range.
Pre-Mosaic, I have not done much testing yet, in part because it's difficult to find them.