Edit (2016-01-14): correct the last paragraph, as IE6+ can still parse HTML5, albeit not display it properly without shims.
<head>, add this code:
<noscript> <meta http-equiv="refresh" content="0; url=www.example.com/no-js-version"> </noscript>
<meta charset="utf-8"> in the
There is a caveat to this: it only validates as HTML5. This means nothing to modern browsers, but it could provide potential issues if you need to validate this as HTML 4 for older browsers (e.g. IE8 or earlier).