What is the preferred way to force ffmpeg (via mpv) to order the streams with video first and audio second?
Wrong stream order can result in playback problems or certain sites rejecting the webm for being malformed.
ovfirst option is deprecated and no longer works.
You can change the stream order afterwards using something like
-map 0:v -map 0:a but it would be best to get the stream order right in the first place.