Alt Text for Accessibility
Images need alternative text (alt text). Alt text is primarily used by people who use screen readers to access websites, applications, and other software. People who use screen readers are usually visually impaired and their impairments could range from complete blindness or to having vision impairment significant enough that reading text on a page is difficult. People who are completely blind will not be able to see an image on a page at all, while people with low vision may not be able to see an image on a page well enough to discern what it is. If an image is conveying new and important information that is not written out somewhere else on the page, a visually impaired reader will not receive the information about that image unless the alt text is specified by the HTML as an attribute.
Other users of alt text include:
People with very slow internet connections. People who live rurally or in developing countries may be used to accessing the internet with a slow internet connection. With alt text, these users can opt to have images turned off so that pages will load faster, and the alt text will then be displayed in place of the image.
People with cognitive disabilities. If a person is easily distracted or has trouble processing large amounts of information, they may turn off images in order to improve their user experience. The alt text will be displayed in place of the image when they have their images turned off.
Not every image needs alt text. Images that are considered decorative may exist on a page for purely aesthetic reasons, or they may be redundant- repeating information that is already on the page as text. Decorative images generally do not need alt text, but informative images do. Informative images are images that convey some kind of information that is not already on the page. A good rule of thumb for determining whether or not an image is informative is to ask yourself if you will be missing information if the image is gone. If the answer is yes, then the image is considered informative.
Some types of images are always informative, so they will always require alt text. Examples of those types of images include images that are links or buttons, images which contain important text, and logos. It is possible that icons can be considered informative images as well. If an icon does not have text directly next to it that describes exactly what the icon is for, it will likely require alt text that describes the icon and, if it is a link or a button, explains where clicking the link or button will take you.
Web site designers are familiar with alt text and accessibility best practices and are skilled at writing appropriate alt text, making sure to prioritize information in alt text, keeping the alt text language concise, and providing appropriate alt text for buttons and icons.