elst, media_time = -1)
This plays plain.mp4 through a plain <video src> element (no Media Source
Extensions). The video track has a single leading empty edit (duration 3.0 s,
media_time = -1) that should delay the picture by 3.0 s; a white flash is at video
media-time 1.0 s and a 1 kHz beep at audio media-time 4.0 s. Applying the offset delays the
flash onto the beep (both at 4.0 s) and makes the total duration 8.0 s; dropping it
leaves the flash at 1.0 s and duration 5.0 s (media length only).
Chromium applies it here (duration 8.0 s, flash at 4.0 s). Firefox does
not (duration 5.0 s, flash at ~1.0 s). Note Firefox does apply the same offset
through MSE (see the MSE demo), so this is a difference between Firefox's own two paths.