In a controlling user agent, the defaultRequest attribute MUST return the default presentation request if any, otherwise null. In a receiving browsing context, it MUST return null.
If set by the controller, the value of the defaultRequest attribute SHOULD be used by the controlling user agent as the default presentation request for that controlling browsing context. If the the document object's active sandboxing flag set has the sandboxed presentation browsing context flag set, the controlling user agent SHOULD act as if the default request is not set for that browsing context. When the controlling user agent wishes to initiate a PresentationConnection on the behalf of that browsing context, it MUST start a presentation using the default presentation request for the controller (as if the controller had called defaultRequest.start()).
The controlling user agent SHOULD initiate presentation using the default presentation request, only when the user has expressed an intention to do so via a user gesture. For example, by clicking a button in the browser.
Note: Some controlling user agents may allow the user to initiate a default presentation connection and select a presentation display with the same user gesture. For example, the browser chrome could allow the user to pick a display from a menu, or allow the user to tap on an Near Field Communications (NFC) enabled display. In this case, when the controlling user agent asks for permission while starting a presentation, the browser could offer that display as the default choice, or consider the gesture as granting permission for the display and bypass display selection entirely.
Note: If a controlling user agent does not support initiation of a presentation connection from the browser chrome, setting defaultRequest will have no effect.
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https://developer.mozilla.org/en-US/docs/Web/API/Presentation/defaultRequest