[w3c/manifest] Confusing use of MAY and SHOULD (#948)

[In section 2.1 Application's name](https://w3c.github.io/manifest/#application-s-name)

> If the name and short_name members are missing or undefined, a user agent MAY fallback to the Document to find suitable replacements for missing manifest members (e.g., using application-name in place of name or short_name). Alternatively, the user agent SHOULD assign a default name (e.g., "Untitled") that follows platform conventions. Alternatively, a user agent MAY allow the end-user to input some text that can serve as the application's name. 

There are three alternatives with a SHOULD and two MAYs mixed.
Does it mean SHOULD phrase takes precedence over the others? (If so, the SHOULD phrase should come first.)

The paragraph also mentioned `undefined` explicitly, which seems inconsistent with other paragraphs in the same section:

> When either the name member or the short_name member is missing, ...

Does this “missing” do exclude `undefined` ?

> When both the name and short_name members are present,...

Does this “present” do include `undefined` ?

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