Alt text (alternative text) is the value of the alt attribute on an HTML img element. It describes the image's content in words, appearing as a substitute when the image fails to load and serving as the description screen readers convey to visually impaired users.
Alt text is a fundamental web accessibility requirement. WCAG (Web Content Accessibility Guidelines) 2.1 mandates that all non-text content provide a text alternative. An image without alt text is effectively an invisible blank for screen reader users.
From an SEO perspective, alt text is equally important. Google Image Search relies heavily on alt text to understand image content. Properly written alt text can drive traffic from image search results. Google's John Mueller has stated that "alt text is one of the most important elements for image SEO."
In the context of URL shortening services, alt text for OGP images (og:image) matters. Thumbnails displayed when sharing links on social media should have appropriate alt text to improve both accessibility and SEO. QR code images should also include alt text like "QR code linking to [destination]."
Good alt text describes the image concisely and specifically. Avoid redundant phrases like "image of" or "photo of" and instead directly state the information the image conveys. For purely decorative images, set an empty alt attribute (alt="") so screen readers skip them. You can find related books on Amazon.