Definitions for "Browser-safe palette"
A colour table containing only 216 colours, used to precisely match the colours of graphics and pictures in cross-platform Web browsers. The remaining 40 colours vary on IBM-compatible and Macintosh computers and are therefore omitted.
Much like an oil painter with her palette of many unique color combinations, each operating system has its own palette. Many computers out there display only 256 colors at a time, and the Macintosh and Windows operating systems reserve about 40 colors out of the 256, leaving 216 available. Netscape Navigator, Microsoft Internet Explorer, and NCSA Mosaic implemented a 216-color palette that won't dither (i.e., vary the pattern of dots in an image) on different platforms and is "browser safe" (in other words, these 216 colors will always look the same, no matter what platform or browser is being used). Theoretically.
These are colours that look the same on both PC and Mac operating systems. By using only the colours in the "browser-safe palette", you can ensure that your colours will look the same on both operating systems.