<script>
var createNewElement = function (tag, id, text) {
    var result = document.createElement(tag);
    result.setAttribute('id',id);
    result.innerHTML = text;
    return result;
};

var runTests = function () {
    document.getElementById("details").appendChild(createNewElement("b", "toadd", "should have bold test"));
};
</script>

<body onload="runTests()">
    <details  id="details" open>
        <summary>summary</summary>
    </details>
</body>