sRGB

sRGB was written in 1996 by Hewlett-Packard and Microsoft to describe the screen almost everyone already had: a CRT monitor in a dim office. It pins three primaries, a D65 white point, and a transfer curve, and that is the whole of it. Because it described the average device rather than an ideal one, it became the value everything defaulted to, and thirty years later it still is. An untagged #1D63D2 in a stylesheet, an untagged JPEG, a colour picked in a design tool with no profile set: all of them mean sRGB, and none of them say so.

The web writes it three ways and they are the same space. #E5342B, rgb(229 52 43) and color(srgb 0.898 0.204 0.169) name one colour; only the notation differs, hex and rgb() counting in 0 to 255 and the color() form in 0 to 1. What none of the three are is linear light. sRGB channel values carry a transfer curve of roughly the 2.2 power, which is why you cannot average two hex codes and get the colour halfway between them, and why every contrast ratio starts by undoing that curve to recover relative luminance.

It is a small space. As a colour gamut it covers about a third of what a person can see, and its reddest red is noticeably duller than a modern laptop can actually produce. Reaching past it is now a one-line decision: color(display-p3 1 0 0) asks for a red outside the sRGB triangle, and oklch() will accept a chroma beyond any display at all. The discipline is to spend the extra range on decoration and keep anything load bearing inside sRGB, because a browser that cannot show the wider value maps it back in and two screens then disagree about what you wrote.

Two naming confusions are worth clearing. First, “RGB” on its own is a colour model, a set of axes with no fixed meaning, while sRGB is that model with its numbers pinned to actual light, which is why the same triple names a different colour in Display P3. Second, the old “web-safe” palette is not this: that was the 216 colours a 256-colour graphics card could show without dithering in the late nineties, a constraint that died with the hardware. sRGB is not a restricted palette. It is the space the rest of the stack assumes when nobody says otherwise.