Simpler Handler

html.html

<a href='video_content'>Show Video</a>
<a href='about_content'>Show About</a>

<div id='video_content'>...</div>
<div id='about_content'>...</div>

js.js

document.addEventListener('click', function(clickEvent) {
  // https://developer.mozilla.org/en-US/docs/Web/API/Event/target
  var target = clickEvent.target;
  
  // https://developer.mozilla.org/en-US/docs/Web/API/Element/tagName
  if (target.nodeName == 'A') {
    // https://developer.mozilla.org/en-US/docs/Web/API/Event/preventDefault
    clickEvent.preventDefault();
    
    document.getElementById(target.href).style.visibility = "visible";
  };
});