AudioContext.outputLatency property is the estimation in seconds of audio output latency. Technically, this is the interval between the time the UA requests the host system to play a buffer and the time at which the first sample in the buffer is actually processed by the audio output device. For devices such as speakers or headphones that produce an acoustic signal, this latter time refers to the time when a sample’s sound is produced.
For compatibility, FireFox already implemented this. From developers' perspective, this property is necessary to coordinate/compensate the latency from the input to the output. Also very useful for synchronization of the video and the audio stream.