Mootools css calc polyfill. Only for relatively simple cases. Usage: util.calc($$('div')[8], 'width', '70% - 40px'); This will yield a cross browser calc. It uses native if it should be supported. Only supporting a few units for now.

css-calc-polyfill.js

//http://caniuse.com/#feat=calc
//union bool str (-webkit-calc, -moz-calc, calc)
//alternatively see last version where i did a version check based on can i use
document.addEvent("domready", function() {//based off https://gist.github.com/Rob-ot/3399053
    Browser.Features.calc = false;//union bool str (-webkit-calc, -moz-calc, calc)
    ["","-webkit-","-moz-","-o-"].some(function(prefix) {
        var $el = new Element('div', {
            styles: {
                width: prefix + "calc(5px)"
            }
        });
        if ($el.style.length > 0) return Browser.Features.calc = prefix + "calc";
    });
});

util.percentToPixel = function(data, par) {
    par = par || $(document.body);
    var size = par.getSize();
    return {
        x: size.x * (data.x * .01),
        y: size.y * (data.y * .01)
    };
};

//https://gist.github.com/megawac/6525074
//http://www.w3schools.com/cssref/css_units.asp
util.calc = function($ele, style, val) {
    // val = val.replace(/(\(|\))/g, "");
    //simple css calc function polyfill
    //polyfill expects surrounded by brackets <val><unit> <operator> <val><unit> => "33% - 20px + 1em"
    //does not support things like "50%/3 - 5px"
    if (Browser.Features.calc) {
        $ele.setStyle(style, Browser.Features.calc + "(" + val + ")");
    } else {
        var old = $ele.retrieve("calc");
        if (old) {
            window.removeEvent("resize", old);
        }
        var split = val.split(" ");
        var op = split.splice(1, 1);
        var resize = function() {
            var expr = val.replace(/(\d+)(\S+)/g, function(match, size, unit) {
                size = size.toFloat();
                switch (unit) { //unit
                    case "%":
                        var data = {};
                        var dir = style.contains("width") ? "x" : "y";
                        data[dir] = size;
                        return util.percentToPixel(data, $ele.getParent())[dir].round(3);
                    case "em":
                        var fsize = $ele.getStyle("font-size").toFloat();
                        return fsize * size;
                        // case "px":
                    default:
                        return size;
                }
            });
            var size = eval(expr); //safe usage - evals '500-20+12' for example
            $ele.setStyle(style, size);
            return resize;
        };
        window.addEvent("resize", resize);
        $ele.store("calc", resize);
        return resize();
    }
}