govuk-frontend/dist/govuk/components/details/template-expanded.html

GOV.UK Frontend contains the code you need to start building a user interface for government platforms and services.

<details id="help-with-nationality" class="govuk-details" open>
  <summary class="govuk-details__summary">
    <span class="govuk-details__summary-text">
      Help with nationality
    </span>
  </summary>
  <div class="govuk-details__text">
    We need to know your nationality so we can work out which elections you’re entitled to vote in. If you can’t provide your nationality, you’ll have to send copies of identity documents through the post.
  </div>
</details>