As far as HTML5 in general goes, offline play is possible. (I've written simple HTML5 games that play offline on iPhones/iPads. Those used HTML5 caching, which is a bit of a nightmare to get working reliably. )
The trouble is that their platform might not allow it easily. They seem to keep most of the game logic on the server, which is good design, but that likely means they wrote the game logic in something other than Javascript. (I'm guessing Java, because during the aborted launch, their captcha generator was a Java library.) That means they will need to port all their game logic, AI's, etc., to Javascript in order to support offline play.
I wouldn't count on this happening anytime soon, given that they have, what, 150 licenses to implement.
Edit: Also, async play can be made to work. I made a post on the Variants forum analyzing the possibilities for async play, and it's a relatively small number of cards that either need to be banned or tweaked. It wouldn't be full Dominion, but it'd be pretty close. Would it be fun? I don't think so, but then again I don't play much async in general.