Images for web publishers and designers are important parts of the site. It can bring a whole new experience to web site visitors.

Search engines, however, cannot understand the content of the images on a page. The technology to read characters in images is here, but it is not yet as well implemented and as easy as text content.

The workaround for this in HTML page is by using the ALT tags.

ALT tags are text descriptions that are used to describe the image on the IMG tags. It actually is an attribute of the IMG tag. By default if the web visitors use a graphically enabled browser, they will see the image instead of the ALT text.

ALT tags look something like this:
>img src=”imagefile.gif” alt=”description of the image”/>

Unless they actually look at the source code, the content of the ALT tag will not be seen by the visitors.

If the web viewers use text-based browser or turn off image in their browser, the content of the ALT tag will be displayed instead.

This tag is especially useful for navigation bar, which consists of links otherwise not clear without description.

Some search engines will factor in the ALT tags, especially for those which serve as links. This provides another opportunity for web publisher to include important keywords and keyphrases on the page. This is beneficial — but not to a large extent — for search engine optimization.

From the accessibility point of view, ALT description provides users with information about what the image is about.

Note: ALT description should exist in all IMG tags. For images that are not important, such as those used for spacers in layouts, you can use null ALT, or empty ALT, as in alt=”".