Alt Tag

An alt tag is the descriptive text attached to an image in HTML through the alt attribute. Browsers show it when the image fails to load, screen readers read it aloud to visually impaired users, and search engines use it to work out what the picture shows.

The name is technically wrong. alt is an attribute on the image element, not a tag of its own, but "alt tag" has stuck as the industry's everyday term. The attribute is also a requirement under the WCAG accessibility standard, because it is the only route a screen reader has into an image.

A useful alt text describes the subject plainly. "Black winter jacket with detachable hood, front view" gives both a screen reader and a crawler something concrete, where "image1.jpg" says nothing at all. Relevant keywords belong there when they genuinely describe the image, but alt text packed with keywords for their own sake reads as spam. Most alt text stays under roughly 125 characters, since some screen readers truncate longer descriptions, and purely decorative images carry an empty alt attribute instead.

Alt tags are among the oldest instruments in SEO, and they still carry weight: Google Images remains a serious traffic source in many industries, and AI models lean on alt text and the surrounding content when judging what a page is about.

Writing alt text well is its own small craft, and the guide to alt tags takes it apart.