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As of today, it is not supported in FireFox. Chrome, Safari however do support it.

by jshacker

HTML

<details>
  <summary>This is what we know!</summary>
  <img src="//i.imgur.com/jkrECes.gif" />
  <p>
    This is Mr. Pool. He is as dead as he can be. Why dead you ask? Well,
    because he cannot die.
  </p>
  <p>
    If you are able to read this without clicking or interacting with it, then
    details doesn't work as expected. Also, are you using FireFox?
  </p>
</details>