JSFiddle - React, Tailwind, and code Playground

HTML

<template id="text">
  <style>
    .text {
      color:red;
    }
  </style>
  <div class="text">Hello</div>
</template>

<template id="page">
  <style>
    :host {
      border:5px solid green;
      display: block;
      margin:20px;
      height:100px;
      width:100px;
    }
  </style>
  This is a page
</template>

<div id="my-body"></div>

JavaScript

class Text extends HTMLElement {
	constructor(){
  	super();
    this.attachShadow({mode: 'open'});
    const template = document.getElementById('text');
    const node = document.importNode(template.content, true);
    this.shadowRoot.appendChild(node);
  }
}
customElements.define('my-text', Text);

class Page extends HTMLElement {
	constructor(){
  	super();
    this.attachShadow({mode: 'open'});
    const template = document.getElementById('page');
    const node = document.importNode(template.content, true);
    this.shadowRoot.appendChild(node);
  }
}
customElements.define('my-page', Page);

const body = document.getElementById('my-body')
const page = document.createElement('my-page')
const text = document.createElement('my-text')

// Not working, element is hidden
page.appendChild(text)
body.appendChild(page)

// Working
//body.appendChild(text)