{{ govuk.details(
    summary = "Summary text",
    content = "Details text"
) }}
<%= govuk.details(
    summary: "Summary text",
    content: "Details text"
) %> 
<details class="gv-c-details">
    <summary class="gv-c-details__summary">
        <span class="gv-c-details__summary-text">Summary text</span>
    </summary>
    <div class="panel panel-border-narrow">

        Details text

    </div>
</details>
{
  "summary": "Summary text",
  "content": "Details text"
}