The `alt` attribute describes an image for anyone who cannot see it: screen reader users, slow connections where the image never loads, and search engines indexing its content. It is a WCAG requirement and an image-SEO input at once. The craft is describing the information, not the pixels: 'bar chart showing orders doubling after launch' serves the reader; 'chart.png' serves nobody.
Decorative images get an empty `alt=""` — present but ignored — because announcing 'image' with no content is worse than silence. The most common real-world failures are missing alt entirely, filenames-as-alt ('IMG_4032'), and keyword stuffing, which turns an accessibility feature into spam.
Related terms
WCAG
The Web Content Accessibility Guidelines — the standard that defines what makes web content usable by people with disabilities; AA contrast is 4.5:1 for body text.
ARIA
Accessible Rich Internet Applications — attributes that tell assistive technology what your markup means when HTML alone cannot.
Semantic HTML
Using elements that mean what they contain — nav, article, button — so screen readers, crawlers and browsers understand the page for free.
Lighthouse
Google's open-source audit tool — scores performance, accessibility, best practices and SEO in a lab environment.
The bench this belongs to
Web performanceA rebuild that loads on a phone on mobile data, in the country your customers are actually in. Core Web Vitals are the measurement, revenue is the point.
