@polymer/polymer/test/unit/custom-style-async-import.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

<!--
@license
Copyright (c) 2017 The Polymer Project Authors. All rights reserved.
This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
The complete set of authors may be found at http://polymer.github.io/AUTHORS.txt
The complete set of contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt
Code distributed by Google as part of the polymer project is also
subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
-->
<custom-style>
  <style is="custom-style">
    html {
      --cs-blue: {
        border : 8px solid blue;
      };
    }
  </style>
</custom-style>
<dom-module id="x-client">
  <template>
    <style>
      :host {
        display: inline-block;
        border : 4px solid red;
        @apply (--cs-blue);
      }
    </style>
    x-client
  </template>
  <script>
    Polymer({
      is: 'x-client'
    });
  </script>
</dom-module>