I am running Mosaico beta 1.0 beta1 with 4.6.22 and 'View in your browser' does nothing. Of course, this is a beta so not surprising. But... is there a way round this? Otherwise quite impressed with Mosaico after a shaky start, but it would be good to have a greater variety of block shapes to add. Anyway, for now the above is the problem that I would love to find a workaround for.
In general, bugs with extensions should be reported on the extensions' github page.
In this case, you may be interested to know that there is a new CiviMail/Mosaico extension nearing completion which will supercede the one you are trying out.