I have a 2017 27" iMac running High Sierra (10.13.6) and Opera Version:66.0.3515.44. Many (not all) videos will not play. In some it will show a preview but then the video window turns black and will not play or show anything. Im feeling that this must be an issue with my OS (too old a version for the Opera version). I hope thats not the case as I will not upgrade my OS for many reasons.