Version: 56
OS: Android

What steps will reproduce the problem?
const e = new BeforeInstallPromptEvent("");
try {
    e.prompt();
    console.assert(false);
} catch (err) {
    console.assert(err instanceof DOMException);
    console.assert(err.name === "NotAllowedError");
}

Source: https://github.com/w3c/manifest/pull/506/files

What is the expected output?
Asserts pass (because it throws NotAllowedError).

This is the error type going into the new spec, so we should follow that.

What do you see instead?
It throws InvalidStateError.

(Also it rejects, rather than throwing, but I filed a separate bug for that:  Issue 658638 .)