<div id="myElement">
    CSS is awesome.
</div>

<style>
#myElement {
    --circle-color: red;
    background-image: paint(circle);
}
</style>

<script>
    CSS.paintWorklet.import('circle.js');
</script>

// circle.js
registerPaint('circle', class {
    static get inputProperties() { return ['--circle-color']; }
    paint(ctx, geom, properties) {
        // Change the fill color.
        const color = properties.get('--circle-color');
        ctx.fillStyle = color;

        // Determine the center point and radius.
        const x = geom.width / 2;
        const y = geom.height / 2;
        const radius = Math.min(x, y);

        // Draw the circle \o/
        ctx.beginPath();
        ctx.arc(x, y, radius, 0, 2 * Math.PI, false);
        ctx.fill();
    }
});

<div id="myElement">
</div>

<style>
#myElement {
    --image: url('#someUrlWhichIsLoading');
    background-image: paint(image-with-placeholder);
}
</style>

<script>
    CSS.registerProperty({
        name: '--image',
        syntax: '<image>'
    });
    CSS.paintWorklet.import('image-placeholder.js');
</script>

// image-placeholder.js
registerPaint('image-with-placeholder', class {
    static get inputProperties() { return ['--image']; }
    paint(ctx, geom, properties) {
        const img = properties.get('--image');

        switch (img.state) {
            case 'ready':
                // The image is loaded! Draw the image.
                ctx.drawImage(img, 0, 0, geom.width, geom.height);
                break;
            case 'pending':
                // The image is loading, draw some mountains.
                drawMountains(ctx);
                break;
            case 'invalid':
            default:
                // The image is invalid (e.g. it didn't load), draw a sad face.
                drawSadFace(ctx);
                break;
        }
    }
});

<h1>
    Heading 1
</h1>
<h1>
    Another heading
</h1>

<style>
h1 {
    background-image: paint(heading-color);
}
</style>

<script>
    CSS.paintWorklet.import('heading-color.js');
</script>

// heading-color.js
registerPaint('heading-color', class {
    static get inputProperties() { return []; }
    paint(ctx, geom, properties) {
        // Select a color based on the width and height of the image.
        const width = geom.width;
        const height = geom.height;
        const color = colorArray[(width * height) % colorArray.length];

        // Draw just a solid image.
        ctx.fillStyle = color;
        ctx.fillRect(0, 0, width, height);
    }
});