Today I have had problems with Safari working with the websites of United Airlines, Tippet Rise Art and Sculpture and PNC Bank. All three websites failed to allow me to register and I was force to switch to Firefox where everything worked.
In the case of United Airlines, they sent me (and my wife at a different email address) messages that were never received. In the Tippet Rise situation I could not register using Safari but did so on Firefox. In the PNC episode, my monthly statement was greyed out and a white bar appeared across the screen. No such thing happened on Firefox.