CSS Painting API

Essentially, the CSS Painting API contains functionality allowing developers to create custom values for paint(), a CSS <image> function. You can then apply these values to properties like background-image to set complex custom backgrounds on an element.

For example:

aside {
  background-image: paint(myPaintedImage);
}

The API defines a worklet that can be used to programmatically generate an image that responds to computed style changes. To find out more about how this is used, consult Using the CSS Painting API.

PaintWorkletGlobalScope

The global execution context of the paint worklet.

PaintRenderingContext2D

Implements a subset of the CanvasRenderingContext2D API. It has an output bitmap that is the size of the object it is rendering to.

PaintSize

Returns the read-only values of the output bitmap's width and height.

The following example creates a list of items with a background image that rotates between three different colors and three widths. In a supporting browser you will see something like the image below.

To achieve this we'll define two custom CSS properties, --boxColor and --widthSubtractor.

The worklet is an external JavaScript file (in this case we've called it boxbg.js) which defines a paint worklet. Using the worklet, we can access CSS properties (and custom properties) of elements:

registerPaint(
  "boxbg",
  class {
    static get contextOptions() {
      return { alpha: true };
    }
    /*
      Retrieve any custom properties (or regular properties,
      such as 'height') defined for the element, and return
      them as an array.
    */
    static get inputProperties() {
      return ["--boxColor", "--widthSubtractor"];
    }

    paint(ctx, size, props) {
      /*
        ctx -> drawing context
        size -> paintSize: width and height
        props -> properties: get() method
      */
      ctx.fillStyle = props.get("--boxColor");
      ctx.fillRect(
        0,
        size.height / 3,
        size.width * 0.4 - props.get("--widthSubtractor"),
        size.height * 0.6,
      );
    }
  },
);

We used the inputProperties() method in the registerPaint() class to get the values of two custom properties set on an element that has boxbg applied to it and then used those within our paint() function. The inputProperties() method can return all properties affecting the element, not just custom properties.

HTML

<ul>
  <li>item 1</li>
  <li>item 2</li>
  <li>item 3</li>
  <li>item 4</li>
  <li>item 5</li>
  <li>item 6</li>
  <li>item 7</li>
  <li>item 8</li>
  <li>item 9</li>
  <li>item 10</li>
  <li>item N</li>
</ul>

CSS

In our CSS, we define the --boxColor and --widthSubtractor custom properties.

body {
  font: 1.2em / 1.2 sans-serif;
}
li {
  background-image: paint(boxbg);
  --boxColor: hsl(55 90% 60%);
}

li:nth-of-type(3n) {
  --boxColor: hsl(155 90% 60%);
  --widthSubtractor: 20;
}

li:nth-of-type(3n + 1) {
  --boxColor: hsl(255 90% 60%);
  --widthSubtractor: 40;
}

JavaScript

The setup and logic of the paint worklet is in the external script. To register the worklet, we need to call addModule() from our main script:

CSS.paintWorklet.addModule(
  "https://mdn.github.io/houdini-examples/cssPaint/intro/worklets/boxbg.js",
);

In this example, the worklet is hosted at https://mdn.github.io/, but your worklet may be a relative resource like so:

CSS.paintWorklet.addModule("boxbg.js");

Result

While you can't play with the worklet's script, you can alter the custom property values in DevTools to change the colors and width of the background image.

• Using the CSS Painting API
• CSS Typed Object Model API
• Houdini APIs