There is a "browser-safe" palette of 216 colors that PC, Mac, and Web-TV platforms support. Known by other names, such as "Web-safe pal ette," "Safety Palette," and "Netscape Palette," these are colors that should not dither on a computer with a color depth set at 256 (8-bit). Any color not in the browser-safe palette will most likely be dithered (a color made up of different colored dots) or be substituted with the nearest nondithered color match. However, UNIX platforms do not use the browser-safe palette. There is very little overlap between the PC and Mac palettes and the UNIX palettes: FF0000, 00FF00, 0000FF, FFFF00, 00FFFF, FF00FF, 000000, and FFFFFF.
The browser-safe palette was first chosen by the developers of the NCSA Mosaic browser and was actually a subset of the Macintosh System palette. Forty colors were reserved for the browser and Windows GUI's, leaving 216 for the browser-safe palette. This was first published as part of the technical notes for Netscape 1.0. All browser-safe colors are in increments of 20 percent, thus all browser-safe colors are combinations of 0, 51, 102, 153, 204, and 255 in RGB and 00, 33, 66, 99, CC, and FF in hexadecimal.
As stated above, a browser-safe color should not dither on a PC or Mac with 8-bit color. However, in reality, that is not always the case. After extensive testing I have found that the amount of video memory a user has, which browser they are using, how the browser is configured under its user preferences, and whether or not the color is being used in a gif or as a background or text color can affect whether or not a browser- safe color will dither even on computers with higher than 8-bit color depth settings.
By the time a browser tries to render a web page, much of the available color palette can be already used up. The operating system reserves some colors for itself, any other opened application may reserve even more colors, the browser itself may reserve colors. Any images loaded into the web page take their share as well. On low color depth systems with a low amount of video RAM, that palette pie gets cut up rather quickly. A browser that tries to allocate an additional color for a nondithered BGCOLOR may fail if it has already allocated as many as it can. It will usually try to map the color to a nondithered color already in its palette, but I have found a few instances of dithering. AOL 4.0 on a PC with 256 color depth and 512 KB of video RAM, a test page with no images, examples of the 140 color names, the 140 color names were mapped to just 32 different colors. For instance, aliceblue, azure, beige, cornsilk, floralwhite, ghost white, honeydew, ivory, lavenderblush, lightyellow, linen, mintcream, oldlace, papayawhip, seashell, snow, white, and whitesmoke were all mapped to just one color: white (FFFFFF). On a PC with 16 bit color and 2 MB of RAM, I.E. 4.01 dithered several color name examples. For instance, antique-white (FAEBD7) used as a table bgcolor, dithered to F8E8D8 and FF80D8, while Netscape 4.08 on the same machine mapped antiquewhite to F8E8D8 (it should be FAEBD7).
There is also a problem with some of the browser-safe colors and WebTV: pure, high intensity, or fully saturated colors tend to be too bright to be used as thin lines or background colors on WebTV (depend ing upon the condition or quality of the TV set being used) can cause screen distortion, audio noise, or, if used as text color, cause text to "bleed." The worse offenders are pure white (FFFFFF) and pure red (FF0000).
Finally, in many browsers, users can also change their browser display preferences: automatic, nearest-color matching, or dither. If they set their user preferences to nearest-color or substitute color, the browser will not dither a non-browser-safe color but instead map it to its nearest browser-safe match.
If you have low contrast between text and background using similar colors, such as dark blue text on a light blue background, all this dithering or mapping can cause your two colors to become even closer together, or worse, become the same color, rendering your text unreadable.
||Title Page| |Basic Template||
||Color| |Dynamic HTML/Layers| |Form Tags| |Frame Tags| |HTML Entities / ISO Characters||
||Image Tags| |Link Tags| |List Tags| |Other Spacing / Layout Tags| |Rule Tags| |Sound Tags||
||Style Sheets| |Table Tags I| |Table Tags II| |Text Tags| |Bibliography / Resources| |Index||