The alt attribute. One of the two easiest to implement but often unused features of HTML. It’s a required attribute of the img tag. What is it? It’s the textual alternative of the image. In other words, if you can’t see the image, it’s what conveys the information — the meaning, you’re unable to see.
Why is it important?
First off, not all users of the Web are able to see.
A person who is blind is completely capable of using the Web through assistive technology such as a screen reader. This is a piece of software that reads out a Web page. Not just the text sighted users would see, but also the semantics of various elements on a page. Text wrapped in strong tags, emphasis tags, headings, language shifts, etc. When a screen reader encounters an image, the image is identified as such, then the alt attribute is read. If an image is used for a link, the link is identified, then the image, then the alt attribute is read.
If no alt attribute is present, the screen reader identifies the image and the user is left to his or her imagination as to what it might be. If you publish a website, it’s somewhat inherent in that action that you want people to satisfactorily browse your site. Alt attributes help make that possible for the non-sighted audience.
A second reason is that non-sighted software programs might visit your site, too.
Search engine spiders and bots. These programs do not contain eyeballs. When they encounter an img tag, without an alt attribute, the exact meaning and purpose of that image is not known. You include an alt attribute and you’ve just enhanced the meaning of the pages on your site. This is additional information to add to your page’s search relevancy and just might assist in better search rankings.