What is CSS Houdini?

Houdini is a set of low-level APIs that give developers direct access to the CSS Object Model (CSSOM) to write custom CSS parsing and rendering.

Traditionally, developers couldn't polyfill CSS. Houdini changes this by allowing JS to hook into the browser's styling, layout, and paint engines. The Paint API, for example, lets you write JS functions that dynamically draw backgrounds or borders, which act natively like CSS.