JSFiddle - React, Tailwind, and code Playground

HTML

<template>
  <style>
    .container {
      border:1px solid black;
    }
    .container>.number {
      color:red;
    }
    .container>.letters {
      color:blue;
    }
  </style>
  <div class="container">
    <div class="number"><content select=".number"></content></div>
    <div class="letters"><content select=".letters"></content></div>
  </div>
</template>

<div id="test1">
  <div class="number">1</div>
  <div class="letters">QUX</div>
</div>

<div id="test2">
  <div class="number">2</div>
  <div class="letters">FOO</div>
</div>

<div id="test3">
  <div class="number">3</div>
  <div class="letters">BAR</div>
</div>

JavaScript

var shadow = document.querySelector('#test2').webkitCreateShadowRoot();
var template = document.querySelector('template');
shadow.appendChild(template.content);