@polymer/polymer/test/smoke/style-props/src/elements-defaults.html

The Polymer library makes it easy to create your own web components. Give your element some markup and properties, and then use it on a site. Polymer provides features like dynamic templates and data binding to reduce the amount of boilerplate you need to

<style is="custom-style">
  html {
    --x-s: {
      display: inline-block;
      margin: 16px;
      border-radius: 4px;
      padding: 2px;
    };
  }
</style>