{{ govuk.button(
    isStart = true,
    text = "Start now"
) }}
<%= govuk.button(
    isStart: true,
    text: "Start now"
) %> 
<button class="gv-c-button gv-c-button--start">
  Start now
</button>
{
  "isStart": true,
  "text": "Start now"
}