<div contenteditable="true">
    <div>There should an empty H1 block below, followed by a normal empty paragraph.</div>
    <div id="div"><br><br></div>
    <div id="div">There should be a single empty paragraph above.</div>
</div>

<script>
div = document.getElementById("div");
window.getSelection().setPosition(div, 0);
document.execCommand("FormatBlock", false, "h1");
</script>