govuk-frontend/dist/govuk/components/input/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-form-group">
  <label class="govuk-label" for="test-name">
    National Insurance number
  </label>
  <input class="govuk-input" id="test-name" name="test-name" type="text">
</div>