Preact + Web Components = <333 Demo: http://www.webpackbin.com/VJyU9wK5W

example.js

import { h, Component } from 'preact';
import Markup from 'preact-markup';
import register from './preact-custom-element';

// just a proxy component: WC -> Preact -> WC
const A = () => <x-b foo="initial foo from <x-a>" />;

// stateful component that can re-render
class B extends Component {
  render(props, state) {
    return (
      <div>
        <label style={{ display:'block' }}>
          foo=
          <input onInput={this.linkState('foo')} />
        </label>
        <x-c foo={state.foo || props.foo} />
      </div>
    );
  }
}

// accepts props (as attributes) from B (<x-b>)
const C = ({ foo }) => (
  <span>I am {'<x-c>'}. I got passed foo = <code>{foo}</code></span>
);

// register stuff (or do it inline above)
register(A, 'x-a');
register(B, 'x-b');
register(C, 'x-c');

// WC on the outside:
document.body.innerHTML = '<x-a></x-a>';

preact-custom-element.js

import { h, render } from 'preact';

const Empty = () => null;

export default function register(Component, tagName) {
	let prototype = Object.create(HTMLElement.prototype);
	prototype._vdomComponent = Component;
	prototype.attachedCallback = prototype.attributeChangedCallback = renderElement;
	prototype.detachedCallback = unRenderElement;
	return document.registerElement(
		tagName || Component.displayName || Component.name,
		{ prototype }
	);
}

function renderElement() {
	this._root = render(
		toVdom(this, this._vdomComponent),
		this.shadowRoot || this.createShadowRoot(),
		this._root
	);
}

function unRenderElement() {
	render(h(Empty), this.shadowRoot, this._root);
}

function toVdom(element, nodeName) {
	if (element.nodeType===3) return element.nodeValue;
	if (element.nodeType!==1) return null;
	let children=[], props={}, i=0, a=element.attributes, cn=element.childNodes;
	for (i=a.length; i--; ) props[a[i].name] = a[i].value;
	for (i=cn.length; i--; ) children[i] = toVdom(cn[i]);
	return h(nodeName || element.nodeName.toLowerCase(), props, children);
}

Jason, Example link above doesn't work anymore, I have create a working example now https://codesandbox.io/s/preact-web-components-z2g9e

@mhadaily thank you very much - your correction helped me a lot.

@RendeCode, perfect, I am glad but keep in mind this example will not work anymore at all because of https://developer.mozilla.org/en-US/docs/Web/API/Document/registerElement (This API was removed from browsers) and you should follow this https://preactjs.com/guide/v10/web-components/