Writing for Accessibility: Alt Tags and Long Descriptions
An alt tag is a brief description of an image. The “alt” in alt tag stands for “alternative” and an alt tag is alternative text—another option to the image. Alt tags should state the type of image and a brief summary of the image. They should not have any unnecessary text. Alt tag text should be approximately four to ten words long. Alt tag text is designed to be brief. The point is to capture the function of the graphic and to express it in terms that make sense.
Every image has an alt tag associated with it. An alt tag must appear for every purposeful image. The alt tags appear on screen with mouse-over, or when the mouse is moved over the image. Assistive technology such as a voice-output screen-reader will not “read” an image but will read the alt tag instead. Text-only browsers display alt tags over the image placeholder.
A long description is a detailed description of an image that supports or adds meaning to the text. Long descriptions are context-specific. The details given depend on how the image supports or supplements the text. Their purpose is to provide content information conveyed by the image so that students who are unable to “read” the image, for whatever reason, still have access to the information relevant to instruction that is conveyed by the image.
Long descriptions are provided whenever an alt tag is not sufficient to convey the content of an image. Long descriptions should be written for each image (map, timeline, picture, chart, graph, photo, etc.) that supports the text or gives additional or new information needed to understand content or topic. A long description should be included whenever an alt tag cannot provide sufficient information about the object and its purpose for inclusion. Remember that long descriptions vary according to learning goals. Try to create a balance between brevity and sufficient information so that every learner can access key content.
Editorial Process Guidelines for Creating Accessible Digital Textbooks. CAST, Inc. 2004