//= require modernizr // Method of allowing calculated values for length units, i.e. width: calc(100%-3em) caniuse.com/#search=calc // By @calvein

Modernizr.addTest('csscalc', function() {

var prop = 'width:';
var value = 'calc(10px);';
var el = document.createElement('div');

el.style.cssText = prop + Modernizr._prefixes.join(value + prop);

return !!el.style.length;

});