px to em with rem support

gistfile1.txt

@function em($pixels, $context: 16) {
    @if ( comparable($pixels, 1px)) {
        @if (unitless($pixels)) {
            $pixels: $pixels * 1px;
        }
        @if (unitless($context)) {
            $context: $context * 1px;
        }
        @return $pixels / $context * 1em;
    }
    @return $pixels;
}