CSS:

.cf:before,.cf:after { content: ""; display: table; }
.cf:after { clear: both; }
.cf { *zoom: 1; }

Sass:

@mixin clearfix {
    @if $legacy-support-for-ie6 or 
    $legacy-support-for-ie7 {
        *zoom: 1;
    }
&:before,&:after {
    content: "";
    display: table;
}
&:after {
    clear: both;
    overflow: hidden;
}
};