An alt atribute is an accompanying command associated with certain HTML and XHTML tags designed to provide a way to associate alternative text with said tags. The most common use of this atribute is in the img tag for the purpose of captioning graphics. Some people call this an alt tag, but this is not accurate because it is not an html tag. The term “tag” has a distinct meaning in web design referring to the commands between angle brackets that do the actual markup in HTML. The alt atribute uses the following syntax:

<img alt="text" src="source url">
<area alt="alt text">
<input alt="text field prompt">

The img tag is only one of several tags that accept the alt atribute. Other tags are “area” and “input.” The img tag displays an image. The area tag displays a link in an image map, and the alt atribute provides the anchor text for the link. An input tag tells the web browser to display a text field and uses the alt text to provide the associated prompt.

Descriptions of graphics have historically been one of the most common uses of the alt atribute. This has been especially useful to people who are visually impaired, who use software to read the screen that is not capable of interpreting a picture. The main purpose of this atribute is to provide a caption rather than to describe the associated image. An example of proper alt text would be “George Washington endured many hardships as General during the revolutionary war.” Actual image descriptions are the job of the title atribute.

Search engines make use of the text provided in the alt atribute. If a web page consists solely or primarily of pictures, the search engine spider is able to crawl the page despite the fact that images by themselves are useless to search engine spiders. The alt atribute is required for the img tag in later versions of HTML, but some web developers put nothing between the quotes. This fails to take advantage of an opportunity for better search engine optimization.

Older web browsers rendered the alt atribute as tooltips, so web developers incorrectly used it to perform the task of a tooltip, which was to provide additional information about an image, such as a description. Newer versions have corrected this problem. The title atribute is now used for this purpose, and the text of this atribute can be seen by hovering the mouse over an image or link.

Web browsers that are not able to display images display the alt text instead. Older browsers always do this, but it is possible to configure more recent versions to display the alt text only instead of the image. This serves the purpose of saving bandwidth because the browser does not have to load the image.