Re: [w3c/manifest] Manifest processing should not be a function of document URL (#668)

There are two problems. The versioned manifest is not the problem that we're dealing with here.

- Problem 1: Sites change their manifest URL every time they update. This means they effectively are publishing a new app every time they update.
- Problem 2: Sites do not specify `start_url` in their manifest. This means effectively you get a different app depending on which page you were on when you installed the site.

> One example of a site that will be affected is weather.com, which has manifests at "versioned" URLs such as https://weather.com/weather/assets/manifest.507fcb498f4e29acfeed7596fe002857.json. If the manifest URL uniquely identifies a PWA then a "new" PWA will be created every time weather's manifest is deployed or moved.

This is already a problem (Problem 1), and my proposed change doesn't make it any worse. Right now, an app is uniquely identified by the tuple (*manifest URL*, *document URL*), so if the manifest URL changes, it's a new app. My proposal is to remove *document URL* from that tuple. It will still be a problem that if the manifest URL changes, it's a new app.

I don't want to talk too much about Problem 1 here. I agree it's a problem but we're talking about it elsewhere (GoogleChrome/lighthouse#2570, GoogleChromeLabs/gulliver#323).

For Problem 2: I noticed weather.com also doesn't have a `start_url`, which means according to the manifest spec, it should create a unique app for each page of the site. **However** it looks like Chrome, at least, considers the manifest invalid if it doesn't have a `start_url`:

> "Site cannot be installed: the start URL in manifest is not valid"

Given that this is the case, it seems it would not be breaking any existing expectations if we made `start_url` mandatory in the manifest spec.

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