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 doesn't match the viewport then skip it
            if ( sheet_media.length && ! window.matchMedia(sheet_media).matches ) continue;
            // get the style rules of this sheet
            rules = [].slice.call(sheet.cssRules);
            // loop the rules
            while ( rule = rules.shift() ) {
                // if this is an @import rule
                if ( rule.stylesheet ) {
                    // add imported stylesheet to the stylesheets array
                    style_sheets.push(rule.stylesheet);
                    // and skip this rule
                    continue;
                }
                else if ( rule.media ) {
                    // add this rule to the stylesheets array since it quacks like a stylesheet (has media & cssRules attibutes)
                    style_sheets.push(rule);
                    // 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);
                }
            }
        }
        return result;
    };
}