Macros using HTML5 are sometimes badly rendered when edited in WYSIWYG mode

This can be reproduced easily with:

{{html clean="false"}}
<details>
  <summary>
    <span class="panel-title">Title</span>
  </summary>
  <div class="panel-body">Body</div>
</details>
{{/html}}

When you edit this in WYSIWYG mode, both the summary and the rest of the details are visible from the start and if you inspect the HTML you'll notice that both the summary and details tags are missing (removed by CKEditor). You're left with the SPAN and DIV.