Squircle rectangle CSS paint worklet

Import the module in javascript.

CSS.paintWorklet.addModule('squircle-rect.js')

For more on paint worklet usage with package managers and bundlers, there's Houdini.how - Using Houdini

Use with paint(squircle-rect) function in css. Control color and radius with --squircle-color and --squircle-radius css variables.

.squircle-rect {
  --squircle-color: black;
  --squircle-radius: 32; /*pixels*/
  background: paint(squircle-rect);
}

Use CSS Custom Paint / Paint Worklets polyfill for better browser support.

<script src="https://unpkg.com/css-paint-polyfill"></script>

Limitations

First off: Pull requests welcome! Let's keep it simple and efficient though, since paint worklets may run quite a lot on a page.

• The squircle shape is an approximations using beziers. My understanding is that the approach is similar to how Figma does it, but quite a simplified version. May improve if I can grasp the math a little better.
• Radius is only in pixels at the moment. Didn't yet figure out how to have it accept any css <length> value like your regular border-radius does.
• Only seems to work as a background-image in Chrome at the moment. Safari and Firefox use the polyfill, and with that, it looks like the squircle can be used anywhere a css <image> type can be used.