Well, so much for that. ffprobe on the resulting mp4 said
Code:
pix_fmt=yuv420p10le
level=150
color_range=tv
color_space=unknown
color_transfer=unknown
color_primaries=unknown
chroma_location=unspecified
whilst the original .webm said
Code:
pix_fmt=yuv420p10le
level=-99
color_range=tv
color_space=bt2020nc
color_transfer=smpte2084
color_primaries=bt2020
I guess I'll need to look at the ffmpeg commandline to see what options go with codec libx265