Safari has now shifted to decoding HTML5 via hardware - which allows for 1080p resolution at streaming speeds on newer machines only. In retrospect this solution seems obvious but it was anything but obvious when nothing worked right because of hundreds of keychain error messages blocking every action. Spent many hours with Apple support early this morning (they were not able to solve this and escalated the problem to engineer status and scheduled a call-back etc) - they were very patient however (kudos) and concluded there were corrupted keychain identification? files - luckily, I resisted all advice to reset keychain and allowed software update (updating back to the future Safari 12) to fix the problem. Warning - Someone recommended reverting back to Safari 11 to allow Prime video to work - I tried this and it caused Safari to no longer recognize or accept my keychain password - or to sync keychain via iCloud - nearly lost all my keychain passwords!
