When we move into Web 2.5, now everybody is on mobile devices. That changes things. We need client-side programming. We also need to adapt to different screen sizes, and we have to deal with very different networking conditions.
At this point, it becomes all about the client—what the client cares about, what the browser cares about, and how this works on tablets and mobile devices. This is where rich user interfaces come in. We program more on the client because I might be disconnected, or intermittently connected, and I still want a good user experience. Because of that, a lot of the programming now lives on the client device itself. It just makes things easier.
That doesn’t mean we have to do it this way. But if we want to embrace tablets and mobile devices, then we push as much as possible onto the client. My website now includes programming like JavaScript and other client-side technologies, instead of having everything happen on the server.
There’s still a server, of course. The server provides the content—the HTML, the CSS, the application itself. But then the question becomes: where is the programming actually happening? Is it on the server, or is it on the client machine?
When you inspect a page and look at the source code, what you’re seeing there—that’s client-side programming.