The pixel has always been the smallest unit in screen-based design. Because it’s been indivisible, it is the concrete unit of measurement among screen-based designers. The phrase “a pixel is a pixel is a pixel” has been adopted to help print designers not used to fixed-screen density understand the concept. Because of this consistency, web designers have adopted pixels over points and other units to build websites.
Now that hardware is changing and pixel densities are growing, pixels are struggling to find relevance as the stable unit they once were. Browser zooming is one thing and has been covered on QuirksMode. But what is a pixel on high resolution devices today? Why does the 640px × 960px iPhone 4 claim to be 320px × 480px in the browser? The truth is that there are two different definitions of pixels: they can be the smallest unit a screen can support (a hardware pixel) or a pixel can be based on an optically consistent unit called a “reference pixel.”
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