The kind of frame it plays.
Its name, unique in the registry: registering the same name again replaces it. Built in: 'mse-fmp4', 'webcodecs', 'image'.
Among the decoders that can play a type, a higher priority is tried first. The built-ins have 0.
Whether it can play frames of this MIME type in this browser, answered at once. A decoder whose exact answer needs an
asynchronous check answers from what it can see now, and calls Failed later if the check says no.
A new decoder, for one player.
One decoder the registry can create.