The Presentation API spec, developed by the Second Screen Working Group, needs to introduce anallow-presentation token to the set of tokens allowed in thesandbox attribute of aniframe element, in order to allow calls to the Presentation API from nested frames, provided the parent browsing context allows it.

The definition of this flag has been in the Presentation API spec for now:
http://w3c.github.io/presentation-api/#sandboxing-and-the-allow-presentation-keyword

As the Presentation API spec is getting ready for publication as Candidate Recommendation, now seems a good time to move the definition to where it should rather be, meaning in the HTML spec.

Note the flag has made its way to the WHATWG HTML spec a few days ago inwhatwg/html@0678e3d

Not sure if you can cherry-pick the commit directly. Happy to prepare the corresponding PR for the HTML5.1 spec otherwise.

(For reference, related issue on the Second Screen Working Group issue tracker:w3c/presentation-api#299)