I'm currently packaging a website as a "Hosted Web App" for the Windows Store.
Is it possible to check if JavaScript, CSS and image files are already on the users devices inside the app package? This way a lot of traffic could be saved for files that don't change much after all.
Of course there would need to be a fallback to the hosted files in case the site is being called in a normal browser.
This is something that we're currently investigating and will be enabling more of in the future. Today you are able to access resources from the app package using the ms-appx-web:/// app protocol to load the resource. You can build this into your website code. We do have a security violation issue when your site is using https (as it definitely should) that we're working on resolving.