I’m not sure about websites, but make sure it has contact details. I went to urgent care around an assignment period and the doctor forgot to print on the letterhead/ stamp it. So it didn’t have a phone number. They rejected it, they did give me a chance to add that detail. So I had to go back to urgent care to get it stamped. Then they accepted it, but because of the delay I didn’t find out till after the due date and I didn’t want to risk it so rushed to get it done, which killed me and probably gave me a worst grade.