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<link rel="author" title="David Grogan" href="dgrogan@chromium.org">
From <a href="https://html.spec.whatwg.org/multipage/rendering.html#the-details-and-summary-elements">html.spec.whatwg.org</a>:

<blockquote>
The details element is expected to render as a block box. The element's shadow
tree is expected to take the element's first summary element child, if any, and
place it in a first block box container, and then take the element's remaining
descendants, if any, and place them in a second block box container.
</blockquote>

&lt;details display:flex> should be ignored. Otherwise details would render as
something other than a block box.
<hr>

<details open>
  <summary>This is the summary.</summary>
  <div>thing 1</div>
  thing 2
</details>