PaintWorkletGlobalScope

class CheckerboardPainter {
  paint(ctx, geom, properties) {
    // The global object here is a PaintWorkletGlobalScope
    // Methods and properties can be accessed directly
    // as global features or prefixed using self
    const dpr = self.devicePixelRatio;

    // Use `ctx` as if it was a normal canvas
    const colors = ["red", "green", "blue"];
    const size = 32;
    for (let y = 0; y < geom.height / size; y++) {
      for (let x = 0; x < geom.width / size; x++) {
        const color = colors[(x + y) % colors.length];
        ctx.beginPath();
        ctx.fillStyle = color;
        ctx.rect(x * size, y * size, size, size);
        ctx.fill();
      }
    }
  }
}

// Register our class under a specific name
registerPaint("checkerboard", CheckerboardPainter);

if ("paintWorklet" in CSS) {
  CSS.paintWorklet.addModule("checkerboard.js");
}

<style>
  textarea {
    background-image: url(checkerboard);
    background-image: paint(checkerboard);
  }
</style>
<textarea></textarea>

@supports (background: paint(id)) {
  background-image: paint(checkerboard);
}