mozGetMatchedCSSRules.js

// polyfill window.getMatchedCSSRules()
if ( typeof window.getMatchedCSSRules !== 'function' ) {
    //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 = [].slice.call(document.styleSheets);

        while ( sheet = style_sheets.shift() ) {
            sheet_media = sheet.media.mediaText;
            // if this sheet is disabled skip it
            if ( sheet.disabled ) continue;
            // if this sheet's media is specified and is NOT all or screen then skip it
            if ( sheet_media.length &&
                ! (~ sheet_media.indexOf('screen') || ~ sheet_media.indexOf('all')) ) continue;
            // get the style rules of this sheet
            rules = [].slice.call(sheet.cssRules);
            // loop the rules
            while ( rule = rules.shift() ) {
                //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);
                }
            }
        }
        return result;
    };
}