Web development explained to a time traveler from ten years ago

Remember Swing, SWT and the likes of wxWidgets? We had to reinvent them for the browser world. Several new UI programming models emerged, which mostly focused on components. We had to find a way to design, build, and test apps while keeping them responsive (a term we use to describe a website that doesn't look like crap on a mobile phone). We also needed to keep it slim -- not everybody has a fast connection, but everybody has a browser in their pockets.