/* text green in IE (both 6 & 7) */
.classname {color: green;}
/* text red in Safari and Firefox */
html:not([dummy]) .classname {color; red}
.classname {color: green;}
/* text red in Safari and Firefox */
html:not([dummy]) .classname {color; red}
Use this one sparingly, but it could be a lifesaver if a site is misbehaving in IE7 and you need to get things working using a fix that you've already applied for IE6. (Note: this one isn't always Opera friendly). If you really need a more thorough, and complicated, hack, have a quick google, but I recommend using this hack as firefighting fix while you get on and re-code your site to behave in IE7 without CSS hacks or filters.