In principle, none. In practice, many websites will check the user agent and flat out refuse service unless it's one of a pre-approved list of implementations,

In principle, none. In practice, many websites will check the user agent and flat out refuse service unless it's one of a pre-approved list of implementations, regardless of whether it would work or not. This has been happening for a long, long time already, for many browser features, and such "standards" pushed forward by the major mobile vendors will only make the problem worse.