[w3c/uievents] May the state of modifier keys influence MouseEvent.button? (#184)

Consider:

```
window.document.addEventListener('mousedown', e => console.log(e.button));
```

On the Mac, with default system settings for modifier keys, depressing the primary mouse button with the control key simultaneously depressed produces output that varies between browsers:

* Chrome and Safari produce `0`
* Firefox produces `2`

Does the spec have an opinion on which behavior is correct, or is this left as unspecified platform-dependent/browser-dependent behavior?

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