This syntax is not standard but is supported by real browsers:
<meta http-equiv="refresh" content="0; https://www.example.com/new-page.html">
It skips the 'url=' part. Htmlunit complains "Malformed refresh string (found ';' but no 'url=')" and fails to handle it.