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The alt attribute is the HTML attribute used in HTML and XHTML documents to specify alternative Plain text ( alt text) that is to be displayed in place of an HTML 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). and text-based web browsers read the alt attribute in place of the image. The text within the alt attribute substitutes the image when as text and makes images more machine-readable, which improves search engine optimization (SEO).
Internet Explorer 7 and earlier render text in alt attributes as tooltip text, which is not compliant with the World Wide Web Consortium (W3C)'s HTML standards. This behavior led many to misuse the alt attribute when they wished to display tooltips containing additional information about images, instead of using the title attribute that was intended for that use. As of Internet Explorer 8, released in 2009, alt attributes no longer render as tooltips on Internet Explorer.
A screen reader such as Orca will read out the alt text in place of the image. A text-based web browser such as Lynx will display the alt text instead of the image (or will display the value attribute if the image is a clickable button). A graphical browser typically will display only the image, and will display the alt text only if the user views the image's properties, or has configured the browser not to display images, or if the browser was unable to retrieve or to decode the image.
The use of descriptions in the alt attribute improves search engine optimization and allows image-specific search engines, such as Google Images, to search for and display relevant images that are used on websites in search results. For non-image search results, the text within the alt attribute is read by search engines the same way that regular text on the page is read.
The W3C recommends that images that convey no information, but are purely decorative, be specified in CSS rather than in the HTML markup. If decorative images are rendered using HTML that do not add to the content and provide no additional information, then the W3C recommends that a blank alt attribute be included in the form of alt="". This makes the page more navigable for users of screen readers or non-graphical browsers by skipping over images that do not convey any meaning. If no alt attribute has been supplied, then browsers that cannot display the image will not overlook the image but instead will read or display the URL or another identifying marker. This creates ambiguity since the user is generally unable to determine from a bare reading of a URL if the image is relevant to the text or if it is a purely decorative element of the webpage. A 2021 Google Lighthouse audit showed that 27% of alt text attributes audited were empty, despite the fact that the majority of those images were non-decorative informational images.
In the United States, there have been several high-profile lawsuits involving the lack of alt attributes on images that cite a violation of the Americans with Disabilities Act (ADA). The United States Department of Justice gives the lack of alt attributes as an example of a barrier to website accessibility. National Federation of the Blind v. Target Corp. was a 2006 class-action lawsuit that alleged that Target.com violated the ADA because the images did not use alt attributes. This lawsuit set a legal precedent in the United States for website accessibility and compliance with the ADA.
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