guideline: Text Alternatives
Provide text for all non-text content.
WCAG 2.0, 2.1, 2.2
Level A
1.1.1 Non-text Content
All images and other non-text content (like icons, charts, audio, CAPTCHAs, or controls) must have a descriptive text alternative that conveys their meaning. Purely decorative content can be hidden from assistive technologies (e.g. using an empty alt attribute).
