Paint Worklet

Paint Worklet, also known as the CSS Paint API, is an effort to enable web developers deeper control of how elements on their page are painted. More details can be found in the spec, but in short the API allows developers to specify a custom JavaScript function for any CSS attribute that can take a CSS <image> type. When the particular part needs to be painted, the JavaScript function is called with a PaintRenderingContext2D that can be drawn into:

paint(ctx, size) { ctx.fillStyle = 'green'; ctx.fillRect(0, 0, size.width, size.height); }

This functionality first shipped in Chrome M65.

Off-Thread Paint Worklet

Off-Thread Paint Worklet (design doc) is an ongoing effort (as of 2019/01) to run the developer-provided paint functions on a different thread than the main rendering loop, and asynchronously from the standard document lifecycle. The goal is performance isolation, to allow smooth animation of Paint Worklet-painted elements even when the main thread is busy. This is considered an architectural improvement project and should not be visible to web developers (other than in improved performance of pages that use Paint Worklets).