A Gecko only polyfill for Webkit's window.getMatchedCSSRules

mozGetMatchedCSSRules.js

// polyfill window.getMatchedCSSRules() in FireFox 6+
if ( typeof window.getMatchedCSSRules !== 'function' ) {
    var ELEMENT_RE = /[\w-]+/g,
            ID_RE = /#[\w-]+/g,
            CLASS_RE = /\.[\w-]+/g,
            ATTR_RE = /\[[^\]]+\]/g,
            // :not() pseudo-class does not add to specificity, but its content does as if it was outside it
            PSEUDO_CLASSES_RE = /\:(?!not)[\w-]+(\(.*\))?/g,
            PSEUDO_ELEMENTS_RE = /\:\:?(after|before|first-letter|first-line|selection)/g;
        // convert an array-like object to array
        function toArray (list) {
            return [].slice.call(list);
        }

        // handles extraction of `cssRules` as an `Array` from a stylesheet or something that behaves the same
        function getSheetRules (stylesheet) {
            var sheet_media = stylesheet.media && stylesheet.media.mediaText;
            // if this sheet is disabled skip it
            if ( stylesheet.disabled ) return [];
            // if this sheet's media is specified and doesn't match the viewport then skip it
            if ( sheet_media && sheet_media.length && ! window.matchMedia(sheet_media).matches ) return [];
            // get the style rules of this sheet
            return toArray(stylesheet.cssRules);
        }

        function _find (string, re) {
            var matches = string.match(re);
            return re ? re.length : 0;
        }

        // calculates the specificity of a given `selector`
        function calculateScore (selector) {
            var score = [0,0,0],
                parts = selector.split(' '),
                part, match;
            //TODO: clean the ':not' part since the last ELEMENT_RE will pick it up
            while ( part = parts.shift(), typeof part == 'string' ) {
                // find all pseudo-elements
                match = _find(part, PSEUDO_ELEMENTS_RE);
                score[2] = match;
                // and remove them
                match && (part = part.replace(PSEUDO_ELEMENTS_RE, ''));
                // find all pseudo-classes
                match = _find(part, PSEUDO_CLASSES_RE);
                score[1] = match;
                // and remove them
                match && (part = part.replace(PSEUDO_CLASSES_RE, ''));
                // find all attributes
                match = _find(part, ATTR_RE);
                score[1] += match;
                // and remove them
                match && (part = part.replace(ATTR_RE, ''));
                // find all IDs
                match = _find(part, ID_RE);
                score[0] = match;
                // and remove them
                match && (part = part.replace(ID_RE, ''));
                // find all classes
                match = _find(part, CLASS_RE);
                score[1] += match;
                // and remove them
                match && (part = part.replace(CLASS_RE, ''));
                // find all elements
                score[2] += _find(part, ELEMENT_RE);
            }
            return parseInt(score.join(''), 10);
        }

        // returns the heights possible specificity score an element can get from a give rule's selectorText
        function getSpecificityScore (element, selector_text) {
            var selectors = selector_text.split(','),
                selector, score, result = 0;
            while ( selector = selectors.shift() ) {
                if ( element.mozMatchesSelector(selector) ) {
                    score = calculateScore(selector);
                    result = score > result ? score : result;
                }
            }
            return result;
        }

        function sortBySpecificity (element, rules) {
            // comparing function that sorts CSSStyleRules according to specificity of their `selectorText`
            function compareSpecificity (a, b) {
                return getSpecificityScore(element, b.selectorText) - getSpecificityScore(element, a.selectorText);
            }

            return rules.sort(compareSpecificity);
        }

        //TODO: not supporting 2nd argument for selecting pseudo elements
        //TODO: not supporting 3rd argument for checking author style sheets only
        window.getMatchedCSSRules = function (element /*, pseudo, author_only*/) {
            var style_sheets, sheet, sheet_media,
                rules, rule,
                result = [];
            // get stylesheets and convert to a regular Array
            style_sheets = toArray(window.document.styleSheets);

            // assuming the browser hands us stylesheets in order of appearance
            // we iterate them from the beginning to follow proper cascade order
            while ( sheet = style_sheets.shift() ) {
                // get the style rules of this sheet
                rules = getSheetRules(sheet);
                // loop the rules in order of appearance
                while ( rule = rules.shift() ) {
                    // if this is an @import rule
                    if ( rule.styleSheet ) {
                        // insert the imported stylesheet's rules at the beginning of this stylesheet's rules
                        rules = getSheetRules(rule.styleSheet).concat(rules);
                        // and skip this rule
                        continue;
                    }
                    // if there's no stylesheet attribute BUT there IS a media attribute it's a media rule
                    else if ( rule.media ) {
                        // insert the contained rules of this media rule to the beginning of this stylesheet's rules
                        rules = getSheetRules(rule).concat(rules);
                        // and skip it
                        continue
                    }
                    //TODO: for now only polyfilling Gecko
                    // check if this element matches this rule's selector
                    if ( element.mozMatchesSelector(rule.selectorText) ) {
                        // push the rule to the results set
                        result.push(rule);
                    }
                }
            }
            // sort according to specificity
            return sortBySpecificity(element, result);
        };
}