What are CSS Houdini APIs?

Answer

CSS Houdini is a set of low-level browser APIs that expose the CSS engine internals to JavaScript, allowing developers to extend CSS with custom behaviors that were previously impossible without JavaScript hacks. Key APIs include: Paint API: define a registerPaint() worklet to draw custom backgrounds, borders, or images directly with the Canvas API. Layout API: create entirely custom layout algorithms (like Masonry). Animation Worklet: create timeline-driven animations that run on the compositor thread. Properties and Values API (CSS.registerProperty()): register custom properties with types, enabling CSS transitions on custom properties. Houdini moves extensibility from "JS-powered class toggling" to "native browser rendering pipeline."