Web Safe colors were the colors which primitive screens were able to display.
There is a general accepted 216 web safe colors.

For CSS designers Web safe colors are the colors which can be written in shorthand like for example,
code #609 is equal to RGB code rgb(102,0,153) or HEX code #660099 or
code #f39 is equal to RGB code rgb(255, 51, 153) or HEX code #ff3399

A lot of browser and CSS preprocessors accept these shorthands and it becomes more
easier to use these colors in design.