This naturally presents accessibility problems for people with visual disabilities. Most authoring guidelines recommend authors to use this feature. HTML 4.0 makes this a required attribute. This is especially critical for images that are being used for image maps. Unlike IMG, object is a container that wraps around the alternative content to be used when the object itself can't be rendered directly.
Many people take a long time to upgrade to newer browsers.
As a result, Websites design their HTML pages to look good on older browsers as well as the latest models. As a result, the IMG element will be in use for a long time to come, and we still need to find a way for authors to make IMG more accessible. This will in most cases be placed in a separate file. People using say speech-based browsers will first read the alt attribute and then choose whether to follow the link provided by the longdesc attribute. We also think this section should be moved in the Browser guideline book, not in the HTML spec.