Text equivalents are used for simple graphics.
A NULL ALT is used in your image tag if you have used graphics for layout.
A 'longdesc' attribute is used for complex graphics that may need more in-depth coverage.
A description link 'D' is used where user agents don't support 'longdesc'.
Graphic file sizes are small for faster downloads.
Client-side image maps are used instead of server-side image maps for image maps or images used as links.