Using bundled packages for offline support is entirely the wrong strategy. The coupling is too tight even before you start trying to use the same format for multiple browsers.
You should just copy Apple and just make it easy to save web pages as local apps, though you might want to explicitly offer with an infobar when there's a manifest available. What they already look for is all you need:
<html manifest="..."> and window.applicationCache
<meta name="application-name"> and <meta name="application-url">
<link rel="icon" sizes="..."> or <link rel="apple-touch-icon">
The capability model for requesting extra permissions is terrific, but it should have been done using <meta> or <link> tags -- reusing their bundle format for extensions was a terrible idea, especially since they're going to have to implement all the DOM-based stuff anyway.
You should just copy Apple and just make it easy to save web pages as local apps, though you might want to explicitly offer with an infobar when there's a manifest available. What they already look for is all you need: