The status of a session's key.
Readonly
The key id.
WebKit- and MS-prefixed EME don't report key ids; for them this is the session's init data, and the status is usable once a key is added.
usable
The key's status, e.g. usable, expired, or output-restricted.
expired
output-restricted
https://w3c.github.io/encrypted-media/#dom-mediakeystatus
The status of a session's key.