why is contenteditable beforeinput event not cancelable?

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Reading this webkit document, It says the beforeinput event when used with contenteditable is cancelable. But I run the code in the document on a chrome browser and the event is not cancelable.

Am I mistaken? How to enable the beforeinput event to be cancelable?

<body onload="setup()">
  <div id="editor" contenteditable>
    <p>This is some regular content.</p>
    <p>This text is fully editable.</p>
    <div class="quote" style="background-color: #EFFEFE;">
      <p>This is some quoted content.</p>
      <p>You can only change the format of this text.</p>
    </div>
    <p>This is some more regular content.</p>
    <p>This text is also fully editable.</p>
  </div>
</body>

<script>
  function setup()
  {
    editor.addEventListener("beforeinput", event =>
    {
      if (event.inputType.match(/^format/))
        return;

      for (let staticRange of event.getTargetRanges())
      {
        if (nodeIsInsideQuote(staticRange.startContainer)
          || nodeIsInsideQuote(staticRange.endContainer))
        {
          event.preventDefault();
          return;
        }
      }
    });

    function nodeIsInsideQuote(node)
    {
      let currentElement = node.nodeType == Node.ELEMENT_NODE ? node : node.parentElement;
      while (currentElement)
      {
        if (currentElement.classList.contains("quote"))
          return true;
        currentElement = currentElement.parentElement;
      }
      return false;
    }
  }
</script>
  

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