Mp4: mp4_start_key_frame directive. The directive enables including all frames from start time to the most recent key frame in the result. Those frames are removed from presentation timeline using mp4 edit lists. Edit lists are currently supported by popular players and browsers such as Chrome, Safari, QuickTime and ffmpeg. Among those not supporting them properly is Firefox[1]. Based on a patch by Tracey Jaquith, Internet Archive. [1] https://bugzilla.mozilla.org/show_bug.cgi?id=1735300