What is the Details/Summary Element?

<details>
  <summary>Click to expand</summary>
  <p>This content is hidden until clicked.</p>
</details>

<details open>
  <summary>Already expanded</summary>
  <p>This content is visible initially.</p>
</details>

<details>
  <summary>What is your return policy?</summary>
  <p>You can return any item within 30 days.</p>
</details>

<details>
  <summary>Do you ship internationally?</summary>
  <p>Yes, we ship to over 100 countries.</p>
</details>

details {
  border: 1px solid #ddd;
  padding: 10px;
}
summary {
  cursor: pointer;
  font-weight: bold;
}