It is necessary for the ESP8266 to close the connection after sending the data, otherwise the browser will time out and complain.
I do that with AT+CIPCLOSE=0, but if I do it too soon after sending the data, the module complains ("busy inet"). I have not found a way to ask the module when it is done transmitting and ready to close the connection, for now I am just waiting. 500 mS seems to do it but I would prefer to have a way to either close the connection automatically after sending the data, or query the module to know when it's ready so that I do not have to wait any longer than necessary.