Different browsers handle css differently or not at all. IE7 for example handles it quite poorly compared to modern browsers.
There are charts you can look up to see what rules which browsers support and which don’t. Here are two links to get you started:
http://www.quirksmode.org/css/contents.html
http://www.webdevout.net/browser-support-css
solved CSS, not working on IE 7 [closed]