That depends on which web interface you are using.
The only one I ever rarely use is the one provided by my ISP, and I only use that if I'm having a problem with my eMail client.
But they are all as different as all web pages tend to be.
But on the one I use, the reply is not a button, and it is not near the body of the message, so for me, it's easiest if I do control + F and type in the word reply and press enter to get to it.
HTH.
Glenn