Web development was much simpler only a few years ago, when we were building HTML pages that included images and photos, and all elements shared the same resolution units. If for example, you aimed at a standard 1024x768 screen, you knew these were exactly the number of pixels available for displaying HTML elements and images.
In 2010, Apple introduced the iPhone 4 with Retina display. In order to simplify things for developers, the logical screen resolution remained the same as previous iPhone models (640x960) while the physical screen resolution was exactly doubled (1280x1920). This means that if, for example, you embed an image tag in your HTML page with width of 200 pixels and height of 300 pixels, and you display a double-size image of 400x600 pixels, the Retina display shows all pixels of the larger image, resulting in a much clearer visual result and without performing browser-side down-scaling.