Spreadsheets had (view-only) offline capability via Gears as far back as 2008 (see http://googledocs.blogspot.co.uk/2008/04/view-your-presentat...) but this was then discontinued in favour of the standards methods. FWIW, they have been helping with the HTML5 storage APIs from the beginning.
If it worked in Chrome outside of the store, it would be more plausible. But requiring the Chrome Store seems to indicate it is not using just standard HTML5 stuff like appCache. The issue of limited storage or not seems insufficient to explain this.
Spreadsheets had (view-only) offline capability via Gears as far back as 2008 (see http://googledocs.blogspot.co.uk/2008/04/view-your-presentat...) but this was then discontinued in favour of the standards methods. FWIW, they have been helping with the HTML5 storage APIs from the beginning.
The HTML5 way of doing offline is AppCache, which Google have used for other products (http://googlecode.blogspot.co.uk/2009/04/gmail-for-mobile-ht...) and are presumably using here (or the alternative FileSystem API - http://dev.w3.org/2009/dap/file-system/pub/FileSystem/).
I suspect the Chrome-only limitation is due to the differences in the way is currently implemented, for instance:
* Until Firefox 26's release yesterday, it would prompt for each site that wanted to use AppCache (https://news.ycombinator.com/item?id=6881368).
* Firefox doesn't handle the no-store directive well
* Chrome Apps can be granted unlimited storage (https://developers.google.com/chrome/whitepapers/storage#unl...)