Colour gamut

Every RGB colour space is defined by three primaries and a white point. The primaries mark the corners of a triangle on a chromaticity diagram, and every colour inside it can be made by mixing them. sRGB, Display P3 and Rec. 2020 are three common gamuts, each larger than the last; MDN's description of the color-gamut media feature states that the P3 gamut includes sRGB and the Rec. 2020 gamut includes P3.

A display's gamut is the triangle its panel can actually reach, usually quoted as a percentage coverage of one of those spaces. A wide-gamut panel only shows wide colour when the content was made in a wide gamut and the operating system and browser pass it through. Most web pages, documents and games are sRGB, and look the same on either kind of screen. Gamut is not HDR: gamut is how saturated colours can be, HDR is how bright and dark the picture can go.

A browser can report which gamut it is given, through the color-gamut media query, and draw colours past sRGB for you to judge. The colour gamut test does both, with pairs of squares for red, green, blue, cyan and green, wider still. It cannot measure the panel: coverage percentage, accuracy and white point need light measured by a colorimeter, so the test never prints a percentage.