Rules, Images, and Multimedia - HTML and XHTML: The Definitive Guide - page 215

several features that make it popular for use in HTML and XHTML doc-
uments. Its encoding is cross-platform so that with appropriate GIF-de-
coding software (included with most browsers), the graphics you create
and make into a GIF file on a Macintosh, for example, can be loaded in-
to a Windows-based PC, decoded, and viewed without a lot of fuss. The
second main feature is that GIF uses special compression technology
that can significantly reduce the size of the image file for faster transfer
over a network. GIF compression is "lossless," too; none of an image's
original data is altered or deleted, so the uncompressed and decoded
image exactly matches its original. Also, GIF images can be easily an-
imated.
Even though GIF image files invariably have the .gif (or .GIF ) filename
suffix, there actually are two GIF versions: the original GIF87 and an
expanded GIF89a, which supports several new featuresincluding trans-
parent backgrounds, interlaced storage, and animationthat are popular
with web authors (see section 5.2.1.2 ). The currently popular browsers
support both GIF versions, which use the same encoding scheme that
maps 8-bit pixel values to a color table, for a maximum of 256 colors per
image. Most GIF images have even fewer colors; there are special tools
to simplify the colors in more elaborate graphics. By simplifying the GIF
images, you create a smaller color map and enhance pixel redundancy
for better file compression and, consequently, faster downloading.
However, because of the limited number of colors, a GIF-encoded image
is not always appropriate, particularly for photorealistic pictures (see the
discussion in section 5.2.1.3 ). GIFs make excellent icons, reduced-color
images, and drawings.
Because most graphical browsers explicitly support the GIF format, it is
currently the most widely accepted image-encoding format on the Web.
It is acceptable for both inline images and externally linked ones. When
in doubt as to which image format to use, choose GIF. [*] It will work in
almost any situation.
[*] We cannot resist the temptation to point out that choosy authors choose GIF.