mkvalidator WRN0C0 First Block for video track #1 (closed) in Cluster is not a keyframe
When running mkvalidator on a file generated by mkvmerge you see a lot of warnings:
WRN0C0: First Block for video track #1 in Cluster at 1573396738 is not a keyframe
What do they mean?
The Matroska specs suggest that each cluster should start with a key frame. The reason behind this suggestion is that this makes seeking a tiny bit faster. However, this is not a strict requirement.
mkvmerge does not really follow this suggestion. There are several reasons why it's not even technically possible to always have a cluster start with a key frame, especially in situations with multiple video tracks or if the timecode scale factor has been lowered. Also long years of practical experience have shown that players can deal with mkvmerge's way of writing clusters just fine.
For these mkvmerge doesn't even try to start each cluster with a key frame.