[1] AirPlay Overview: Opting Into or Out of AirPlay ( (2013-01-13 13:13:01 +09:00 版)) http://developer.apple.com/library/ios/#documentation/AudioVideo/Conceptual/AirPlayGuide/OptingInorOutofAirPlay/OptingInorOutofAirPlay.html

[2] AirPlay Overview: Opting Into or Out of AirPlay ( (2014-06-16 04:53:25 +09:00 版)) https://developer.apple.com/library/ios/documentation/AudioVideo/Conceptual/AirPlayGuide/OptingInorOutofAirPlay/OptingInorOutofAirPlay.html

[3] Safari 7.0 (2015-11-04 11:53:32 +09:00 版) https://developer.apple.com/library/safari/releasenotes/General/WhatsNewInSafari/Articles/Safari_7_0.html#//apple_ref/doc/uid/TP40014305-CH5-SW7

This attribute supersedes the older x-webkit-airplay attribute, which is now deprecated; however, the old attribute values will still be respected, denying wireless playback if an element has x-webkit-airplay="deny".

[4] Why not use the Safari API? · Issue #1 · w3c/remote-playback (2015-11-05 17:21:10 +09:00 版) https://github.com/w3c/remote-playback/issues/1#issuecomment-153695287

x-webkit-airplay=deny and x-webkit-wirelessvideoplaybackdisabled are used by three dozen websites.
x-webkit-airplay=allow and x-webkit-airplay (no attribute) is used significantly more. Three order of magnitude more. Though, these attributes are a no-op.