A web browser is similar to the old-style 3270 terminals too. All the mainframe server did was send bytes down to the terminal (well, terminal controller), the

A web browser is similar to the old-style 3270 terminals too. All the mainframe server did was send bytes down to the terminal (well, terminal controller), the terminal itself would do all of the display formatting including field validation, then would send the user's data back up to the server. It's not quite the same since a web browser can run arbitrary code, but the browser is still used to display data to the user and return user's input to the server, while the back-end server does data storage and processing -- for example you wouldn't implement Google's search engine on client-side alone, all of the heavy lifting is done by Google's servers.