Edit in JSFiddle

Result
HTML
JavaScript

<body>
  <div class="wrapper">


  </div>
</body>

const createDOMFromSring = (domstring) => {
	const div = document.createElement('div');
  div.innerHTML = domstring;
  return div
}

class UpVoteButton {
  render() {
  	this.elem = createDOMFromSring(`
  	<button class='upvote-btn'>
    <span class='upvote-text'>Upvote</span>
    <span><i class="fa fa-thumbs-up" aria-hidden="true"></i></span>
    </button>
  `)
  return this.elem;
  }
}

class DownVoteButton {
  render() {
  this.elem = createDOMFromSring(`
    <button class="downvote-btn">
    <span class="downvote-text">Downvote</span>
    <span><i class="fa fa-thumbs-down" aria-hidden="true"></i></span>
    </button>
    `)
    return this.elem
  }
}

// as render() method now returns an Object of HTMLDivElement
// it cannot be appended like innerHTML, instead use DOM
// API to append it
const wrapper = document.querySelector(".wrapper");
const upVoteButton = new UpVoteButton();
wrapper.appendChild(upVoteButton.render())

const downVoteButton = new DownVoteButton();
wrapper.appendChild(downVoteButton.render())