<details>
  <summary>
    The quick brown fox ...
    <span>[SPOILERS]</span>
  </summary>
  The quick brown fox <em>jumps over the lazy dog!</em>
</details>
details[open] summary {
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}
summary::-webkit-details-marker {
  display: none;
}
summary {
  list-style-type: none;
}
span {
  color: red;
  cursor: pointer;
}

/* ignore the rest */
body {
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  background-color: white;
  border: .5em solid #555;
  bottom: 0;
  font-family: sans-serif;
  font-size: 1.1em;
  left: 0;
  margin: 0;
  padding: 1em;
  position: absolute;
  right: 0;
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}
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