ffmpeg safe pixel format specification

By default, the solution pack generates MP4s which only playback successfully in Chrome. To create a "safe" MP4 which plays back in all browsers, ffmpeg recommends the following switch:

-vf format=yuv420p

Line 32 of my derivatives.inc now looks like this:
$command = "$ffmpeg_executable -i $archival_path -f mp4 -vcodec libx264 -vf format=yuv420p -strict -2 -acodec aac -movflags +faststart $out_file";

In summary, this setting recommendation produces an MP4 file which successfully plays on all browsers.