[devote]

function queryStyleRule(selector) {
    var sheets = document.styleSheets,
        j, i = sheets.length,
        rules = [];
    for( ;i--; ) {
        try {
            rules = sheets[i].cssRules ? sheets[i].cssRules : sheets[i].rules ? sheets[i].rules : [];
        } catch(_e_) {
            rules = [];
        }
        for(j = rules.length; j--; ) {
            var sel = rules[j].selectorText.replace(/(^|[^:]):(after|before)/i, '$1::$2');
            if (sel.toLowerCase() === selector) {
                return rules[j];
            }
        }
    }
    var style = document.documentElement.firstChild.appendChild(document.createElement('style'));
    try {
        selector = selector.replace(/::(after|before)/, ':$1');
        if (style.sheet) {
            style.sheet.insertRule(selector + '{}', 0);
            return style.sheet.cssRules[0];
        } else if (style.styleSheet) {
            style.styleSheet.addRule(selector, ' ', 0);
            return style.styleSheet.rules[0];
        }
    } catch(_e_) {
        style.parentNode.removeChild(style);
    }
    return null;
}


queryStyleRule('body::after').style.content = '"Hello World!"';;

alert(queryStyleRule('body::after').style.content);