Sometimes you need to detect an user agent, something like a browser switch. In Typoscript you can do it very simple.
For Opera, providing a Opera specific css file.
[browser = opera] page.includeCSS { file5 = fileadmin/css/opera.css } [END]
For Internet Explorer and version equal or smaller than version 7
[browser = msie] && [version =< 7.0] page.includeCSS { file5 = fileadmin/css/ie7.css } [END]