Re: [w3c/manifest] Add a way to query whether there is a UA-provided back button (#693)

Hi all! Wanted to jump in and try to provide a related use cases and problems we ran into with the Twitter PWA, in the hopes it informs the current discussion:

1) I would like to go one step further in this proposal and point out **it's important to us to differentiate the TYPE of back button provided**. For instance, the back windows provides in the nav bar is not useful. But if Android indicates it provides back as a hardware button, that may be more noteworthy.

For instance, right now in the Twitter PWA, we actually go to great lengths to make back behavior sensible. Only "pages" have "routes", things like dropdowns or sheets do not. This is a small distinction, but consider the UI patterns across devices. On Android, a user expects the back button to close an open sheet, and NOT to navigate back a literal page. On a desktop browser, when I hit back, I expect the last page to come up, not just some random dropdown to close.

This is a small distinction, but you can see that the actual differentiator in behavior is if the back came from a hardware vs software button/shortcut. This is something Android natively does, but is not exposed in websites.

2)  For CSS vs JS, I get that it can be exposed in either. I don't think doing feature detection via CSS is as common place as has been argued yet, even `display: standalone` CSS detection is tedious at best. We use it only to try and figure out if the user currently has Twitter "installed". I'd much prefer to have `navigator.features.installed` or something like that which is direct and to the point. Roundabout way of saying, ultimately we'll follow whatever, but **JS for long-term consistency** with other feature detections?

Long, but hopefully that's all helpful. I'll try and think more about it and circle back with more.

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