atRule = function(prop) {
    var length = prefixes.length;
    var cssrule = window.CSSRule;
    var rule;

    if (typeof cssrule === 'undefined') {
      return undefined;
    }

    if (!prop) {
      return false;
    }

    // remove literal @ from beginning of provided property
    prop = prop.replace(/^@/, '');

    // CSSRules use underscores instead of dashes
    rule = prop.replace(/-/g, '_').toUpperCase() + '_RULE';

    if (rule in cssrule) {
      return '@' + prop;
    }

    for (var i = 0; i < length; i++) {
      // prefixes gives us something like -o-, and we want O_
      var prefix = prefixes[i];
      var thisRule = prefix.toUpperCase() + '_' + rule;

      if (thisRule in cssrule) {
        return '@-' + prefix.toLowerCase() + '-' + prop;
      }
    }

    return false;
  }
atRule returns a given CSS property at-rule (eg @keyframes), possibly in
some prefixed form, or false, in the case of an unsupported rule

@memberof Modernizr
@name Modernizr.atRule
@optionName Modernizr.atRule()
@optionProp atRule
@access public
@function atRule
@param {string} prop - String name of the @-rule to test for
@returns {string|boolean} The string representing the (possibly prefixed)
valid version of the @-rule, or `false` when it is unsupported.
@example
```js
 var keyframes = Modernizr.atRule('@keyframes');

 if (keyframes) {
   // keyframes are supported
   // could be `@-webkit-keyframes` or `@keyframes`
 } else {
   // keyframes === `false`
 }
```