Providing alternative text (ALT Text) is one of the easiest accommodations to provide, but unfortunately, it is either missing or inadequate on most pages. Alternative text is used to provide a textual alternative to images, media, applets, and any other non-text content. The focus here will be on images, graphs, charts, and a briefly on videos.
Alt text provides:
Screen readers and search engines cannot analyze images, graphs, etc. to determine content and function. This is the information that the alternative text must provide and can be done in two ways:
The description should be clear, but succinct. If it cannot be succinct, then another linked page from longdesc attribute (long description) is used.
It is important to consider the context of the page when writing the Alt text. For example, the same photo of Pensacola Beach would have two different Alt attributes depending on whether it was used on a tourist web site, or with an article on the BP oil spill.
Charts, graphs and maps use visuals to convey complex images to users. But since they are images, these media provide serious accessibility issues to colorblind users and users of screen readers.