Writing ALT Text

Alan Flavell has written everything you need to know to write good ALT text. This is a condensed version.

Types of users who need ALT text

There are two types of users who will see ALT text:

Users who can load images but aren't at the moment. (To save bandwidth, e.g.)
Users who can't load images (using a text browser, blind, etc.)

These users need slightly different information from the ALT text. The first group wants to know whether to bother downloading that image. The second group needs to get (as far as possible) the information contained in the image.

What to write depends on what the image is

There are 4 categories of images, which need to be treated differently:

Eye Candy (Bullets, decorations, logos, etc.): These don't add anything to the content, and should generally use ALT="". Nobody wants to see (or hear)

little yellow bullet science
little yellow bullet instruments
little yellow bullet launch vehicle
little yellow bullet operations
Navigation icons: Use the appropriate text (Next, Previous, Index, etc.)
Supplemental or interesting images: Here is where the art comes in. Try to describe the most important idea(s) that the image conveys (but try not to exceed 60-80 characters!). This is not the same thing as a description of the image. For example, if you're discussing convection in outflows, don't use
ALT="HST photo of Eta Carinae"
when the point you want to make is
ALT="Eta Carinae looks like it's boiling"
Essential images: Here it hardly matters what ALT text you use. If some images are essential and some aren't, you can say that in the ALT text.

Read the full article on the subject

Again, for more detail read Alan Flavell's article.

Document author: Kevin R. Boyce (email: Kevin.R.Boyce@gsfc.nasa.gov)
This page was last modified on 15-Jan-98 at 4:08 PM