What is the function of the 'alt attribute' in an HTML image tag?

The 'alt attribute' in an HTML image tag serves primarily to provide alternative text that describes the content of the image. This description is critical for accessibility; it helps visually impaired users who rely on screen readers to understand what the image conveys. When an image cannot be displayed due to a slow connection or an error, the alt text serves as a placeholder, allowing users to understand the context of the image.

Additionally, the alt attribute is important for search engine optimization (SEO) as it gives search engines context about the image, contributing to better indexing and helping improve the visibility of the webpage in search results. Properly using the alt attribute therefore enhances both accessibility and SEO, making it an essential part of web development best practices.