Image does not have a LONGDESC attribute that points to an HTML file providing a detailed description of the image.

Make sure the image presents information that is not included in the page or in the text equivalent of the image (that is, in its ALT attribute).

If describing the image will add information not present in the text of the page, then you need to provide a description of the image. The amount of information in the image and the context in which it will be used will determine how detailed the description should be.

However, since not many browsers at the moment support the LONGDESC attribute, you may also want to place a so-called D-link near the image. A D-link is a normal textual link with a label "D" pointing to an HTML page providing a full image description. Example:

    <IMG src="chart.gif" alt="chart of beverages distribution"
    longdesc="chart.html"><A href="chart.html">D</A>

Placing a rich textual description such as a caption close to the image is another viable solution. In such a case, you will not need the LONGDESC attribute or the D-link.

The ALT attribute cannot be used to provide a full-length description of the content of an image. Think of a diagram, a histogram, a chart, or a product picture in an online catalogue. To describe its content, you need more flexibility.

The LONGDESC attribute can be used to provide a long description of the associated image. By including the attribute LONGDESC="any_HTML_file" in your IMG tag, you can link the image to the HTML file containing a formatted description of the image. The long description (unlike the ALT attribute) can contain HTML code, with links to other resources, formatting instructions, and more.

However, since not many browsers at the moment support the LONGDESC attribute, you may also want to place a so-called D-link near the image. A D-link is a normal textual link with a label "D" pointing to an HTML page providing a full image description. Example:

    <IMG src="chart.gif" alt="chart of beverages distribution"
    longdesc="chart.html"><A href="chart.html">D</A>

Placing a rich textual description such as a caption close to the image is another viable solution. In such a case, you will not need the LONGDESC attribute or the D-link.

Adding a good alternative description to images is very important today because many users can't take advantage of graphics. They include users of cellular phones, PDAs, and browsers installed in cars.