allowfullscreen is an attribute, much like title, src, but a custom one.
If xml was the subject here, then it could be possible to have custom attributes, custom elements.
In html world, this isn’t possible, you have a DTD which describes elements and their attributes. The validator uses this DTD and signals when aliens are spotted.
Yeah, don’t expect valid markup from Youtube – they pretty much crap all over the specification so badly it’s nothing short of a miracle their websites even work in the first place.
The allowfullscreen custom attribute doesn’t appear to make a difference (at least not at the current time)? Remove it and you still get the ‘full screen’ option in the player.