I felt that comment my bones. Why would anyone possibly have the need to know actual presentation timestamp and how that corresponds to actual realtime? Evidently, no one working on WebRTC has had to synchronise data streams from varying sources before with millisecond accuracy.
I was doing a demo for a video stabilisation using a webcam and IMU module in the browser. It turns out the latency between video->rtc->browser and sensor->websocket->browser are wildly different and not constant. The obvious solution would be to send UTC timestamps for the sensors data and synchronise in browser. Not possible, the video has no UTC timestamp reference. When you have control of both sides of the WebRTC pipe, you can do fun things like send the UTC timestamp of the start of the stream, but this won’t solve browser jitter. It worked well enough for a POC but the entire solution had to be reengineered.