third_party/WebKit/LayoutTests/editing/style/designmode.html

<script>
if (window.testRunner)
     testRunner.dumpEditingCallbacks();
</script>
<p>This tests to make sure that rich editing commands like Bold can be performed on text in a document in design mode.</p>
<div id="test">This text should be bold.</div>

<script>
document.designMode = "on";
var s = window.getSelection();
var e = document.getElementById("test");
s.setBaseAndExtent(e, 0, e, e.childNodes.length);
document.execCommand("Bold");
</script>