Pure JavaScript を使用して CSS 継承ツリーを検査するにはどうすればよいですか?

<style type="text/css">
    p { color :red; }
    #description { font-size: 20px; }
</style>

<p>

function css(el) {
    var sheets = document.styleSheets, ret = [];
    el.matches = el.matches || el.webkitMatchesSelector || el.mozMatchesSelector 
        || el.msMatchesSelector || el.oMatchesSelector;
    for (var i in sheets) {
        var rules = sheets[i].rules || sheets[i].cssRules;
        for (var r in rules) {
            if (el.matches(rules[r].selectorText)) {
                ret.push(rules[r].cssText);
            }
        }
    }
    return ret;
}

["p { color :red; }", "#description { font-size: 20px; }"]