As Joe Clark wrote in his classic book Building Accessible Websites:
It would not be remiss to hope that a long description conjures an image - the image - in the mind's eye, an analogy that holds true even for the totally blind.
To write a long text description ask yourself, if there is no description, what will the user miss?
Long Text Alternatives Good Practices
- Describe what you see/what is visually evident.
- Describe from general to specific.
- Describe objectively and accurately. Don't describe motivations or intentions.
- Be descriptive. Try to use vivid, imaginative language.
For further details consult the National Center for Accessible Media's Effective Practices for Description of Science Content within Digital Talking Books.
Using a Link to a Description
A link below or near an image may be used to provide a long description within the document itself or to link to a separate page. This is simple method as everyone will have access to the it.
Linking to separate document can also provide efficiency and scalability making authoring and maintenance easy wherever instances of the same image are used in multiple locations. It is analogous to the power of external style sheets.
Using on Page Description with Location Info in the alt Attribute
When a long description is provided on the same web page as an image, its location can be described using the alt attribute of the image. The location information needs to be clear and accurate to help users locate the content. The
syntax is:
<img
alt="Number of subscribers. Described under the
heading: 'Graph Description: Subscriber Increase'"
src="graph.png">
[…]
<h4>Graph Description: Subscriber Increase</h4>
[…]
Other Methods
The W3C provides more options for providing long descriptions.