The alt attribute is the HTML attribute used in HTML and XHTML documents to specify alternative text (alt text) that is to be displayed in place of an element that cannot be rendered. The alt attribute is used for short descriptions, with longer descriptions using the longdesc attribute. The standards organization for the World Wide Web, the World Wide Web Consortium (W3C), recommends that every image displayed through HTML have an alt attribute, though the alt attribute does not need to contain text. The lack of proper alt attributes on website images has led to several accessibility-related lawsuits.

The alt attribute is used to increase accessibility and user friendliness, including for blind internet users who rely on special software for web browsing. The use of the alt attribute for images displayed within HTML is part of W3C's Web Content Accessibility Guidelines (WCAG). Screen readers and text-based web browsers read the alt attribute in place of the image. The text within the alt attribute substitutes the image when copy-pasted as text and makes images more machine-readable, which improves search engine optimization (SEO).