<details open="open">
  <summary>Label</summary>
  <p>someText</p>
</details>

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

details {
  summary {
    cursor: pointer;
  }
  
  p {
    background-color: green;
    max-width: max-content;
    padding: 0 1em;
  }
}