A sample project demonstrating how to load web components with pure Javascript.

1. Create your component html and css files.
2. Add a class for each component to components.js as follows (replace COMPONENT_CLASS_NAME with a name for your component's class):

class COMPONENT_CLASS_NAME extends HTMLElement {
  constructor() {
    super();
    this.innerHTML = COMPONENT_CLASS_NAME._html;
  }
}

3. Add a call to the loadView() function for each component inside the loadComponents() function of components.js.
4. Call the loadComponents() function form your Javascript as follows:

import loadComponents from "./components.js";

document.addEventListener("DOMContentLoaded", () => {
  loadComponents().then(() => {
    start();
  });
});

function start() {
  // The rest of your Javascript.
}

Timing is important here. The DOM must first finish loading, then loadComponents() is called and allowed to complete, then you can call a start() function for the rest of your Javascript.

Here is a blog post covering how this project can be integrated as part of a larger automation & control system. https://www.learnavprogramming.com/html5-part-i/

Thanks to Anton Karsten for helping think this through.