[es-dev-server] CustomStyleInterface polyfill is missing

When serving to legacy browsers, the ShadyCSS.CustomStyleInterface polyfill is not loaded. This means external CSS Variables cannot be used to style elements.

<style>
  my-custom-element {
    --my-prop: red;
  }
</style>

<my-custom-element>
  <!-->begin: shadow<-->
  <style>
    :host {
      color: var(--my-prop, blue);
    }
  </style>
  <slot></slot>
  <!-->end: shadow<-->
  Colour should be red.
</my-custom-element>

<script>
  ShadyCSS.CustomStyleInterface.addCustomStyle(document.querySelector('style'));
</script>

[LarsDenBakker]

With https://github.com/open-wc/open-wc/pull/1292 there is an option to configure to load this

[daKmoR]

and it’s released 🎉