Informative images must not be CSS-only (css-image-informative)
Images conveying information must not be added only via CSS background-image
Informative image via css:
In this example, an image containing the text The five boxing wizards jump quickly is applied as a background image using CSS. This text is visually important but has no text alternative, violating WCAG 1.1.1.
The image contains the phrase: “The five boxing wizards jump quickly.”