Usability is sometimes known as Accessibility. Some people surf with images turned off, others must use disability software. Don't turn these visitors away. If your navigation interface is composed of images, include alt tags. It's also a good idea to provide text links as an alternate method of navigation. For serious usability coding, you can insert hidden pixels with alt tags in your HTML that will be detected by disability software. This technique is to redirect blind users to other portions of the web site designed specifically for them. For Government web sites, Accessibility is a Requirement.