Web safe colors are a set of 216 colors that were selected to be consistently displayed on all computer systems with 8-bit displays, which were common in the early days of the web. These colors were chosen to ensure that any color displayed on a computer monitor would appear the same across all platforms and browsers.
The 216 web-safe colors consist of 6 levels of red, green, and blue (RGB) values, where each level represents a multiple of 51 (0, 51, 102, 153, 204, and 255). By limiting the color palette to this fixed set of colors, designers could be sure that their intended colors would display correctly on all devices and that the colors would not dither or appear distorted due to variations in color representation on different monitors.
However, with the advent of modern displays with greater bit-depth and color gamut, the need for web-safe colors has largely diminished. Nowadays, designers have more freedom to choose from a wider range of colors without worrying too much about cross-platform compatibility issues.