consider adding ISOBMFF media timeline mapping

[jpiesing]

Although it's a little outside the scope of this document, I think it would be helpful if there could be a clear, simple & unambiguous definition of the HTML media timeline for ISOBMFF. HTML defines the media timeline as follows;

Establishing the media timeline: if the media resource somehow specifies an explicit timeline whose origin is not negative (i.e. gives each frame a specific time offset and gives the first frame a zero or positive offset), then the media timeline should be that timeline. (Whether the media resource can specify a timeline or not depends on the media resource's format.) If the media resource specifies an explicit start time and date, then that time and date should be considered the zero point in the media timeline; the timeline offset will be the time and date, exposed using the getStartDate() method.

A clear statement of exactly how to interpret the above for ISOBMFF+MSE would avoid misunderstandings and enable unit tests. It would help with important features like ad reporting.

A clear and simple description is particularly important for people integrating desktop browser code onto the operating systems and hardware used in media consumption devices.