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Without useful alternative text for images, many web pages become difficult or even impossible to navigate, and users are left "feeling their way in the dark."
To prevent this problem, you must provide alternative text for elements such as images and other multimedia objects. The alternative text is usually presented instead of the image by a browser with a voice output system, or on a Braille display used by many people who are blind. It is also common for users with low-quality connections to request the alternate view, because text is transmitted much faster than images.
The alternative text should replace the image in function as well. For example, if you use the image as an icon that links to a reference point, then the title of the reference itself is a good alternative text.
There are cases where an image is used for purely visual effect, and does
not add any information value to the page. In this case, it is appropriate to
use a blank alternative text, although you must still define it by entering
two double quotes for the alternative text (for example,
alt="").
When the image conveys important information that is not available from
the text of the document, you must provide a description of the information.
The longdesc option (Attributes > longdesc)
enables you to link a description to the image. This description can be a URI
(a separate document), or named part of the current document.
To view how a page might be presented without the images, use Show Alternate (Views >Show Alternate). If the page is spoken by a voice output system, then the alternate view might simply be read out.