in your html
site.less .opacity { opacity: 0.5 }
.corners {
border-radius: 3px;
}
rendered as site.css .opacity { opacity: 0.5; -moz-opacity: 0.5; filter: alpha(opacity=50); }
.corners {
border-radius: 3px;
-moz-border-radius: 3px;
-webkit-border-radius: 3px;
}