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Opacity is the inverse of transparency. In CSS, opacity can be set using the opacity property (which affects the entire element and its children) or using alpha-channel color values like rgba() and hsla(). Opacity is essential for creating layered visual effects, overlay patterns, glass-morphism designs, and subtle background tints. When compositing semi-transparent colors, the visible result depends on both the foreground color and whatever is behind it, following the alpha compositing formula defined by Porter and Duff.