https://www.w3.org/TR/css-backgrounds-3/
CSS Backgrounds and Borders Module Level 3
backgrounds and borderslevel 3cssmodule
https://developer.mozilla.org/en-US/docs/Web/CSS/Guides/Backgrounds_and_borders
CSS backgrounds and borders - CSS | MDN
The CSS backgrounds and borders module provides properties for adding backgrounds, borders, rounded corners, and box shadows to elements.
backgrounds and borderscssmdn