index.html

<!doctype html>
<html>
  <head>
    <script defer src="https://unpkg.com/es-module-shims@latest/dist/es-module-shims.js"></script>
    <script type="importmap-shim">
      {
        "imports": {
          "lit-html": "https://unpkg.com/lit-html@latest/lit-html.js?module",
          "lit-element": "https://unpkg.com/lit-element@latest/lit-element.js?module",
          "lit-html/lit-html.js": "https://unpkg.com/lit-html@latest/lit-html.js?module",
          "lit-html/lib/shady-render.js": "https://unpkg.com/lit-html@latest/lib/shady-render.js?module"
        }
      }
    </script>
    <script type="module-shim">
      // you can also set a script src with type "module-shim"
      import { LitElement, html, css } from 'lit-element';

      class XComponent extends LitElement {
        static get styles() {
          return css`
            :host {
              color: red;
            }
          `;
        }

        render() {
          return html`
            Hello from lit-element!
          `;
        }
      }

      customElements.define('x-component', XComponent);
    </script>
  </head>
  <body>
    <x-component></x-component>
  </body>
</html>