<script>
if (window.testRunner)
     testRunner.dumpEditingCallbacks();
</script>
<div style="border:1px solid black" contenteditable="true">
<div id="item1">Foo<br></div>
</div>

<script>
var s = window.getSelection();
var p1 = document.getElementById("item1");
s.setPosition(p1, 0);
document.execCommand("FormatBlock", false, "pre");

</script>