We currently have an inline script tag in the page template that adds the 'js-enabled' class
<script>
document.body.className = ((document.body.className) ? document.body.className + ' js-enabled' : 'js-enabled');
</script>
This is for usability reasons: we don't want to have to wait for the GOV.UK Frontend JS file to load before the correct classes and styling is applied - this approach avoids the flash of content for instance hiding the toggleable content of details and accordion components.