The WakeLockSentinel object has a new attribute called `released` that indicates whether a sentinel has already been released. Its value is initially false, and it changes to true once a "release" event is dispatched.
The new attribute helps web developers know when a lock has been released so that they do not need to keep track of them manually. See https://github.com/w3c/screen-wake-lock/issues/272 for the original discussion.