Details

The native <details> and <summary> elements are styled as an accordion.

<details>
  <summary>Click to expand</summary>
  <p>Hidden content revealed on click.</p>
</details>

Accordion group

Stack multiple <details> elements and they automatically merge borders:

<details>
  <summary>Section one</summary>
  <p>Content for section one.</p>
</details>
<details>
  <summary>Section two</summary>
  <p>Content for section two.</p>
</details>
<details>
  <summary>Section three</summary>
  <p>Content for section three.</p>
</details>