Alt text is required for all meaningful, functional, and informative images to ensure web accessibility for users with visual impairments, allowing screen readers to convey the content. It is a legal requirement under the ADA and Section 508. Good alt text is concise, accurate, and context-dependent, avoiding phrases like "image of". Decorative images should use empty alt="" tags.

Key Requirements and Best Practices:

Necessity:

All images must have an alt attribute, even if it is empty for decorative images (alt="").

Purpose:

It provides a text equivalent for users who cannot see the image.

Quality:

Descriptions should be short, concise, and accurately convey the meaning, usually in a few words or a sentence.

Context:

Avoid repeating surrounding text; focus on why the image is included.

Functional Images:

If an image is a link or button (e.g., a magnifying glass for search), the alt text must describe the action (e.g., "Search").

Complex Images:

Graphs or diagrams may require longer, more detailed descriptions in the surrounding text or a linked file.

Further Reading on Alt Text

WebAIM

W3C WAI

Section 508