Placeholder audits

[paulirish]

I think this is a good list for now:

1. Manifest's start_url is in cache storage for offline use
2. Service worker makes use of push notifications, if appropriate
3. Tap targets are appropriately sized for touch
4. Payment forms marked up for with [autocomplete] attributes
5. Login forms marked up with [autocomplete] attributes
6. Input fields use appropriate [type] attributes for custom keyboards
7. Content scrolls at 60fps
8. Touch input gets a response in < 150ms
9. App is interactive without jank after the first meaningful paint

There are, of course, many more, but I think these sit at the top priority-wise.