MSE does not apply an empty-edit (elst, media_time = -1) presentation offset

The video track carries a single leading empty edit (an elst entry with media_time = -1, media_rate = 1, duration 3.0 s) that delays the picture by 3.0 s; the audio has none. A white flash is at video media-time 1.0 s and a 1 kHz beep at audio media-time 4.0 s, so applying the offset lands the flash exactly on the beep (both at presentation time 4.0 s). Dropping it leaves the flash at 1.0 s, 3 seconds before the beep.

Safari: WebKit fixed this in trunk (bug 316870 - the mediasource-sequencemode-append-buffer.html WPT). A shipping stable Safari still shows the pre-fix "dropped" result until the fix rides into a release; use Safari Technology Preview or a WebKit Nightly to see it applied.

Watch for the flash, then listen for the beep. Each player prints its two event times below it. In Chromium both players beep at the same instant (4.0 s), so you hear a single beep, and both timelines show it; the audio is fine in both. What differs is the video: the MSE player paints its flash 3 s early.
Click "Start".
MSE (appendBuffer): the subject of this report
idle
Direct (video.src): non-MSE control
idle

The verdict reads the MSE player's own signals: video SourceBuffer.buffered.start and the painted flash frame's mediaTime (requestVideoFrameCallback). Chromium and Firefox disagree on these identical bytes; see the README for the spec question this raises.