govuk-frontend/dist/govuk/components/phase-banner/template-default.html

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

<div class="govuk-phase-banner">
  <p class="govuk-phase-banner__content">
    <strong class="govuk-tag govuk-phase-banner__content__tag">
      Alpha
    </strong>
    <span class="govuk-phase-banner__text">
      This is a new service - your <a href="#" class="govuk-link">feedback</a> will help us to improve it.
    </span>
  </p>
</div>