Re: [me-media-timed-events] Add recommendation for events with unknown end time (#39)

I agree with https://github.com/w3c/me-media-timed-events/issues/39#issue-418823597 that the case of unbounded duration needs to be supported.

I also agree with https://github.com/w3c/me-media-timed-events/issues/39#issuecomment-470967005 that we need to handle the case of when an event end time goes from undefined to defined.

However, I think:
1. there are more related requirements:
- event end time goes from undefined to defined (e.g. a sports game, after the undefined game time elapses, goes from undefined to a defined remaining time for post game interviews.)
- event end time goes from defined to undefined (e.g. award show running overtime.)
- event end time going from one defined time to another defined time (e.g. regularly scheduled one hour newscast extended by 30 minutes due to breaking news or one hour program cut short by unscheduled government speech)
2. a sentence in PR #46 moves from requirements to implementation:
> It should also be possible to update the cue's end time, should the end time subsequently become known.

This is one possible implementation of the above requirements. There are other possible implementations:
- define `TextTrackCue.endTime` = `infinity` as equal to the `HTMLMediaElement.duration`. This makes some sense because the [resource selection algorithm](https://html.spec.whatwg.org/#concept-media-load-algorithm) destroys all text track cues when the `HTMLMediaElement.duration` is destroyed. This also handles all requirements without ever changing `TextTrackCue.endTime`.
- Since there is an existing [event on duration change](https://html.spec.whatwg.org/#event-media-durationchange), JavaScript could change TextTrackCue.endTime on event-media-durationchange.

I'm not arguing for or against any of the implementations, but suggesting that the existing "It should be possible..." sentence be replaced by the list of 3 requirements in (1) above, since IG Notes need to stay as requirements.

3. These requirements also need to be added to the [DataCue Explainer](https://github.com/WICG/datacue/blob/master/explainer.md), which I believe doesn't include any of these requirements.

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Received on Tuesday, 21 May 2019 23:29:39 UTC