Sass (scss) rem function and mixin. Based on and improved upon: https://github.com/bitmanic/rem

_rem.scss

//
// Sass rem function and mixin
//  https://gist.github.com/branneman/6762906
//  Based on and improved upon https://github.com/bitmanic/rem
//

// Baseline, measured in pixels
//  The value should be the same as the font-size value for the html element
//  If the html element's font-size is set to 62.5% (of the browser's default font-size of 16px),
//  then the variable below would be 10px.
$baseline-px: 10px;

// Generates the css property in px and also in rem when !$old-ie
@mixin rem($property, $px-values) {
    #{$property}: $px-values;
    @if not $old-ie {
        #{$property}: rem($px-values);
    }
}

// Returns the rem equivalent for the given px value(s)
@function rem($px-values) {

    // Convert the baseline into rems
    $baseline-rem: $baseline-px / 1rem * 1;

    // If there is only one (numeric) value, return the property/value line for it.
    @if type-of($px-values) == "number" {
        @return $px-values / $baseline-rem;
    } @else {

        // Create an empty list that we can dump values into
        $rem-values: ();
        @each $value in $px-values {

            // If the value is zero or not a number, return it
            @if $value == 0 or type-of( $value ) != "number" {
                $rem-values: append($rem-values, $value);
            } @else {
                $rem-values: append($rem-values, $value / $baseline-rem);
            }
        }

        // Return the property and its list of converted values
        @return $rem-values;
    }
}

example.scss

html {
    font-size: 10px;
}
body {
    @include rem(font-size, 18px);
}
li {
    border-top: rem(1px) solid $linkblue;
}