We can't have online multiplayer unless we have containers, for each ordered, debugger, function, and a lot of other basic (as in base, not as in easy/simple) stuff.
Besides, online multiplayer for HTML5 isn't even a thing that exists. We have websockets supported only by firefox and chrome, there's no UDP yet (though they're working on it, as well as p2p: check this document out
, it isn't even past the draft stage). We need UDP or we can't have anything even remotely close to realtime.
My pick is editor stuff (function/or/else/foreach ordered/etc) then containers, then debugger.