It should be well-known (though perhaps isn't) that when Marketo rewrites your offsite links, the redirect from the Mkto-hosted branding domain (f.e. click.example.com) back to www.example.com requires JavaScript. It sets document.location, a primitive method that works fine, but only if the user's browser supports JS. Which sucks because that means you may be flat-out losing .5-1% of users, and even if they copy-and-paste the rewritten links they will see only a white page.
The above is all confirmed by Marketo support, btw.
Support suggested I open a discussion to see how/if any of you are dealing with this usability gap.