title: IntersectionObserver undefined for Internet Explorer
body: <p>I have an IntersectionObserver to lazy load images etc., where <code>el</code> is the HTML Element and <code>offset</code> a number. </p>
  
  <pre><code>const observer = new IntersectionObserver((entries) =&gt; {
    if (entries[0].isIntersecting === true) {
      el.srcset = this.image;
      observer.unobserve(el);
    }
  }, {threshold: [0], rootMargin: this.offset + 'px'});
  
  observer.observe(el);
  </code></pre>
  
  <p><strong>This observer works fine on every browser, but not on IE11.</strong> Debugging it get the following error returned:</p>
  
  <blockquote>
    <p>"Intersection Observer" is undefined</p>
  </blockquote>
  
  <p>Which makes the images not load at all. Is there a solution for IE? Since I suppose it's not supported on IE, but shouldn't the normal behavior be to execute the observer's code initially then? </p>