A Pixel Identity Crisis: 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.”