A shift in the way we develop for digital devices. The mantra used to be ‘graceful degradation’ – design for what you expect people to use, then let your application gracefully degrade for older browsers, and smaller screens. Generally speaking the richest application, and the first to design for, was on the desktop.
A more recent approach is mobile first. Start with core content, design for the smallest screens and least capable browsers, then progressively enhance the experience onto larger more capable devices, browsers and screens. My intro from Mobile First by Luke Wroblewski.