I just realized today that IE 6 class names must not start with something else other then [a-zA-Z]. So don't you dare name your classes like _class1 cause it will not work in IE6.
It works well in Firefox 2.0, IE7, opera and safari and that will just make it harder when you'll try to discover the problem.
I know this seems like a lame mistake and something any designer should know, but well I'm not really a designer and some things you never know unless they hit you.
but you can start a class in IE6 with a number and in Firefox you can’t; anyway IE6 can be a pain in the ass sometimes.
AFAIK w3c specs say you can’t start ca class name with a number so I guess IE is selective about what part of a standard it implements. I would not start a class name with a number anyway… it just doesn’t seem natural.