Merge pull request Modernizr#685 from emilchristensen/master

Add feature detect for nth-child()

feature-detects/css/nthchild.js

@@ -0,0 +1,43 @@
+/*!
+{
+  "name": "CSS :nth-child pseudo-selector",
+  "caniuse": "css-sel3",
+  "property": "nthchild",
+  "tags": ["css"],
+  "notes": [
+    {
+      "name": "Related Github Issue",
+      "href": "https://github.com/Modernizr/Modernizr/pull/685"
+    },
+    {
+      "name": "Sitepoint :nth-child documentation",
+      "href": "http://reference.sitepoint.com/css/pseudoclass-nthchild"
+    }
+  ],
+  "authors": ["@emilchristensen"],
+  "warnings": ["Known false negative in Safari 3.1 and Safari 3.2.2"]
+}
+!*/
+/* DOC
+
+Detects support for the ':nth-child()' CSS pseudo-selector.
+
+*/
+define(['Modernizr', 'testStyles'], function( Modernizr, testStyles ) {
+  // 5 `<div>` elements with `1px` width are created.
+  // Then every other element has its `width` set to `2px`.
+  // A Javascript loop then tests if the `<div>`s have the expected width
+  // using the modulus operator.
+  testStyles('#modernizr div {width:1px} #modernizr div:nth-child(2n) {width:2px;}', function( elem ) {
+    Modernizr.addTest('nthchild', function() {
+      var elems = elem.getElementsByTagName('div'),
+      test = true;
+
+      for (var i = 0; i < 5; i++) {
+        test = test && elems[i].offsetWidth === i % 2 + 1;
+      }
+
+      return test;
+    });
+  }, 5);
+});