Creates a rule.
@param {CSSStyleSheet} styleSheet The StyleSheet to create the rule in as returned from {@link #createStyleSheet}.
@param {String} selector The selector to target the rule.
@param {String} property The cssText specification eg `"color:red;font-weight:bold;text-decoration:underline"`
@return {CSSStyleRule} The created rule
function(styleSheet, selector, cssText) {
            var result,
                ruleSet = styleSheet.cssRules || styleSheet.rules,
                index = ruleSet.length;

            if (styleSheet.insertRule) {
                styleSheet.insertRule(selector + '{' + cssText + '}', index);
            } else {
                styleSheet.addRule(selector, cssText||' ');
            }
            CSS.cacheRule(result = ruleSet[index], styleSheet);
            return result;
        }