ALT tags provide a text alternative to an image.
They are a way to “describe” an image to those who can not see the image. The most important function of an ALT tag is to explain to a blind user what an image is displaying. Search engine crawlers also use ALT tags to decipher what an image is or what it is representing.
If you have images on your web pages that are not described by an ALT tag your HTML is not valid and you are not following this (and other) guidelines.
What should go in a Alt tag?
It is recommended to “use text that fulfills the same function as the image”. When deciding upon the text you use to describe your images it very important that you keep the people who are unable to see your images in mind.
A great example of this is …
If a ‘question mark’ icon or image is used to represent the help page, the alternative text should be ‘help’ or something similar.
Alt tags that are full of keywords are not following this guideline and are disruptive for blind users. It is often not understood that when a blind person is surfing the web they are normally using software that reads aloud the entire page. Can you imagine how irritating it would be to have to listen to an ALT tag that is “stuffed” with keywords?
There is ample information available on the web concerning the proper use of ALT tags.