Modernizr Test 'nthchildn'

css-nthchildn.js

// Test for nth-child(...n) support
Modernizr.testStyles(" #modernizr div:nth-child(3n){width:10px;} ", function(elem, rule){
	var bool = false, divs = elem.getElementsByTagName("div");
	if (divs.length == 7){
		var test = window.getComputedStyle ? function(i){
			return getComputedStyle(divs[i], null)["width"] == "10px";
		} : function(i){
			return divs[i].currentStyle["width"] == "10px";
		};
		bool = !test(0) && !test(1) && test(2) && !test(3) && !test(4) && test(5) && !test(6);
	}
	Modernizr.addTest("nthchildn", bool);
}, 7);

Polyfill for CSS property nthchild for IE7-8 uses Modernizr

USAGE

if (!Modernizr.nthchildn) {  
  $('.span4:even').addClass('margless');
}

If not support for nthchild then add class to elements