Welcome to http and css! It's not a problem with http or css, it's just that different browsers will display some things differently.
Browsers are not all equal. Most browsers support most of the things you can do, but some do not support some of the things. IOW, you have to fiddle around with your formatting and sometimes make compromises to get the best rendering in as many browsers as you want.
Me, I'm lazy. I use IE6 (which doesn't support some of the things that IE7 supports, BTW). If it looks good in IE6 then however it looks in Firefox or Opera or IE7 will just have to do.