Right. You can do this now by loading the page in a background tab, leaving the tab open and then going offline. Works great.

Or you could reimplement newsgroups (but with less interoperability) as an SPA, which in theory would work great (once you work out the kinks) but in practice would suffer from all kinds of little issues in the from-scratch reimplementation of everything. For example, printing would probably forever be a wishlist item in the backlog.