This is a demo site for CSS3. For most stuff you need Safari 4+, Chrome, Firefox 4+ or Opera 10.5x to make these demos work.
For 3D stuff, you need Safari 5, Chrome 10+ or the WebKit Nightly build.
Make your window skinnier until the message gradually appears.
Or on a mobile device, rotate it to landscape mode.
Make your window wider until the message gradually disappears.
Or on a mobile device, rotate it to portrait mode.
This effect makes use of CSS3 @media queries, which is the most prominent method of creating a responsive layout. This basic example should get your gears turning.