What is Alt Text?

Glossary

Alt text is a short written description of an image, stored in its alt attribute, that screen readers read aloud and search engines use to understand the picture.

Alt text (alternative text) is a brief written description attached to an image on a web page. People who use screen readers hear it instead of seeing the picture, browsers show it if the image fails to load, and search engines read it to work out what the image shows.

How alt text is added to an image

In HTML, every img element can carry an alt attribute, for example alt="Red brick bakery storefront with a striped awning". People often call this an "alt tag", but strictly it is an attribute of the image tag, not a tag of its own. Website builders and CMSs usually expose it as an "alternative text" or "description" field when you upload or select an image.

A screen reader such as VoiceOver or NVDA announces the image and then reads the alt text. If the attribute is missing entirely, many screen readers fall back to reading the file name, which is how users end up hearing "IMG underscore 4471 dot jpeg".

Writing good alt text: examples

Describe what the image contributes in that spot, in one plain sentence. Some cases:

Product photo: "Navy linen shirt with mother-of-pearl buttons, front view" rather than "shirt".
Linked logo in the header: describe the destination, such as "Harbour Plumbing home page".
Chart: give the takeaway, and put the full figures in nearby text or a table.
Decorative flourish or background texture: use an empty attribute, alt="", so screen readers skip it.

Why it matters for a small-business website

Text alternatives for non-text content are the very first success criterion in WCAG, and missing alt text is one of the most frequently cited issues in accessibility complaints and lawsuits under laws like the ADA in the United States. Google's image SEO guidance also says it uses alt text, along with the surrounding page, to understand images, which helps them appear in Google Images. Good alt text is part of solid on-page SEO, but it should be written for people first.

Common alt text mistakes

Starting with "image of" or "picture of". Screen readers already announce that it is an image.
Stuffing keywords. "Best cheap plumber Toronto plumber" helps nobody and can look spammy.
Leaving the attribute off decorative images. Use an empty alt so they are ignored, rather than omitting it.
Putting important text inside images. If a banner says "20% off this week", that wording must appear in the alt text or, better, as real text on the page.
Copying the caption word for word. Visitors then hear the same sentence twice.