Some web video streaming sites have cryptocurrency miners embedded in them, or adverts from highly untrustworthy as networks. In either case, that extra garbage on the page can generate extreme CPU usage, pushing the phone up to the point where it's overheat protection kicks in. Do you get extreme heat in other circumstances, such as with YouTube, or Netflix? If it is only the random web streaming sites, then it is the individual sites abusing your CPU.
This I observed while using Facebook from chrome.. even if I play games of 15 to 20 mins I feel more heat.. in device monitor it shows above 45.. and if you leave phone idle of 5 to 10 mins it will reduce heat.