Responsive design
A viewport, or the section of the screen where your browser is, comes in all kinds of sizes. Some are very small, such as the screen of your smartphone, and some can be very large. When I looked at my old website on a screen a lot larger than the one I was used to work on, it was so ugly that I decided to completely redo it using a responsive design.
Designing a web page with a fixed width specified in pixels today really is out of the question, as is making several versions of your website to accommodate all these sizes.
In responsive design—I am always tempted to call it responsible design—one does not start off with a canvas of, let us say, 960 px wide and build a site that way, chopping it into fixed size <div> blocks. As soon as the view port is smaller than 960px, a part of the site would not be visible and, maybe worse, when the screen is really big, everything around your 960x wide rectangle will look boring and empty. This is simply not done...
