In #150, @domenic wrote the following. Creating an issue so it doesn't get lost.
Note that (if it has multi-implementer interest) the https://wicg.github.io/picture-in-picture/#htmlvideoelement-extensions section should migrate to the HTML Standard, linking to the algorithms defined in this separate specification. This would also allow the chance to fix several issues with the current specification due to insufficient integration (e.g. the use of EventHandler attributes on HTMLMediaElement instead of GlobalEventHandlers, or the way that autoPictureInPicture, disablePictureInPicture, onenterpictureinpicture, andonleavepictureinpicture have no definitions).