Web-safe colors
The web-safe color palette (also called the browser-safe color palette) is an obsolete color palette of 216 standard web-safe colors.
Specifically, a web-safe color is one in which the red, green, and blue values all belong to the set {0x00, 0x33, 0x66, 0x99, 0xcc, 0xff}, where 0x00 and 0xff are the minimum and maximum intensities. There is no well-defined colorspace for these values, though it can be assumed to be some rough approximation of sRGB.
Discussion
In the early days of the web, operating systems often ran in a 256-color (paletted) video mode. When a web browser was the foreground application, it would typically set 216 of the available colors to the web-safe palette.
For web developers, the significance of web-safe colors was that if you had a choice of what colors to use on your web site, you might prefer to use web-safe colors. This would increase the chance of a web browser displaying them accurately, and without dithering.
The web-safe palette became obsolete as the number of computers using 256-color video modes dwindled to insignificance.
