chromium/third_party/blink/web_tests/editing/inserting/typing-tab-designmode.html

<body>
<script>
if (window.testRunner)
    testRunner.dumpAsText();
        
document.designMode="on";

var e = document.body;
var s = window.getSelection();
s.collapse(e, 0);

if (window.eventSender) {
    eventSender.keyDown('\t');
    eventSender.keyDown('\t', ["shiftKey"]);
}
</script>
Test to see if tabs work in designMode.  There should be two tabs inserted.  One for the tab key, and one for shift-tab.<br>

</body>