<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title>Cerebro</title>
    <link rel='stylesheet' id='cerebro-theme' />
    <script>
      (function() {
        if (!process.env.HOT) {
          const link = document.createElement('link');
          link.rel = 'stylesheet';
          link.href = '../dist/main.css';
          document.write(link.outerHTML);
        }
      }());
    </script>
  </head>
  <body>
    <div id="root">
    </div>
    <script>
      (function() {
        const script = document.createElement('script');
        script.async = true;
        script.src = (process.env.HOT) ? 'http://localhost:3000/dist/main.bundle.js' : '../dist/main.bundle.js';
        document.write(script.outerHTML);
      }());
    </script>
  </body>
</html>

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title>Cerebro</title>
    <link rel='stylesheet' id='cerebro-theme'>
    <script>
      (function() {
        if (!process.env.HOT) {
          const link = document.createElement('link');
          link.rel = 'stylesheet';
          link.href = '../dist/main.css';
          document.write(link.outerHTML);
        }
      }());
    </script>
  </head>
  <body>
    <div id="root">
    </div>
    <script>
      (function() {
        const script = document.createElement('script');
        script.async = true;
        script.src = (process.env.HOT) ? 'http://localhost:3000/dist/main.bundle.js' : '../dist/main.bundle.js';
        document.write(script.outerHTML);
      }());
    </script>
  </body>
</html>

Change at the end of the link tag to match W3C convention
Change at the end of the link tag to match W3C convention

Fixed the link tag to match W3C convention as mentioned here:
https://www.w3.org/TR/2011/WD-html5-author-20110705/the-link-element.html