For the record, synchronization use cases and requirements have already
been discussed at length in this IG in the past, see e.g.
http://www.w3.org/2011/webtv/wiki/Media_APIs/Use_Cases#10._.22Use_Case_Ten_.E2.80.93_Sync_Among_Browser-_and_non-Browser_Devices.22
but I don't think there has been any practical follow up on that for luck
of a critical mass of contributors to the work.
If there is a renewed interest in doing something my recommendation would
be to keep discussing on this list to identify what we are trying to
achieve and if we need a spec, and if the answer to that is positive, to
setup a CG to refine the extension proposal before submission to a WG,
unless the conclusion is really limited to a few small changes to the HTML5
spec in which case it may be easier to just report bugs against the HTML5.1
spec.
So in short, let's keep discussion going and will see what's the most
appropriate follow up.
/g