Why Use Long Text Alternatives?
Some types of complex images such as charts, graphs, and pictures can convey a significant amount of information visually or provide a specific sensory experience to a sighted person. These images can expand on statements made in a document; enhance understanding; or act as the central medium of information. Providing long text alternatives helps convey this information.
When to Use Long Text Alternatives
Long text alternatives can provide for rich, expressive
documentation of a visual image and should be used when
alt is insufficient to embody the visual qualities of
To help decide whether a long text alternative is necessary turn off images in your browser and ask yourself:
What Long Text Alternatives Provide in Contrast to
Screen readers read out the value of an
alt attribute automatically. The old saying goes "a
picture is worth a thousand words". While this is true, that
doesn't mean everyone wants to hear all one thousand words at once
or at this particular moment. Stuffing a long description into an
alt attribute would force a screen reader user to
listen to it.
In contrast the
longdesc attribute provides users of screen readers such
as JAWS the ability to pause, step outside of the page flow to
obtain a description of a complex image. The difference is akin to
glancing at a sophisticated chart or painting (
alt functionality) versus studying it (
longdesc functionality). Sighted users, have that choice (to glance or
study). Long text alternatives offer that choice to users with
Ability to Utilize Structured Markup
It is impossible to use links or structural markup with an
alt attribute as its value only allows string text.
In contrast a long description can utilize headings, tables, lists, etcetera which can facilitate better communication and provide critical user functionality as discussed in the structure eClass.
Ability to Use a Separate Document
Using one global document to provide long descriptions for multiple instances of the same image is a powerful, portable, re-usable technique. It provides 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.