Alternative text (sometimes incorrectly called ‘alt tags’) is text intended to describe something – typically images, audio and video – for those who can only read text. Typically this includes:
- Users with visual impairment, who have their webpages read out aloud to them
- Search engines
In particular, because search engines can’t understand images, audio and video like humans can, they are unable to index or find content hidden inside without ‘alternative’ text to look at. Including alternative text therefore is good SEO practice.
Whenever images, audio and video are used on the web, a text equivalent should be included. In most cases, this text equivalent is invisible to most users, but is essential to people with visual impairments and search engines. Consider the following image:
An image such as this should include a text equivalent, in this case “Apples” or “Two apples, one sliced in half” would be appropriate.
Alternative text is not required to explain every media in exact detail. From Wikipedia:
altattribute does not always have to literally describe the contents of the image. Keep in mind the purpose and context of the image and what would be useful to someone who cannot see it. The alt attribute is supposed to be an alternative for the image, usually stating its purpose. For example, an image of a warning sign should not have alt text “a triangle with a yellow background, black border and an exclamation mark”, but simply “Warning!”—unless, of course, the alt text’s purpose is to show what the warning symbol actually looks like.
To signal to automated validators like Insites that alternative text has been considered but rejected as unnecessary, it is common to specify an empty alt attribute, e.g.
<img alt=""> or
<img alt>. Some may consider this bad practice.
How it is coded
<img src="image.png" alt="This is alternative text">
Alternative text is tested by:
- Alternative text test – checks whether alternative text is specified, and useful
- Spelling test – checks whether alternative text is spelled correctly
- Alt attribute on Wikipedia