chromium/content/test/data/accessibility/html/design-mode.html

<!--
@BLINK-ALLOW:nonAtomicTextFieldRoot=true
@BLINK-ALLOW:editable*
@BLINK-ALLOW:richlyEditable*
@BLINK-ALLOW:htmlTag*
-->
<p>
  Hello
</p>
<script>
  document.designMode = 'on';
</script>