Detect modern browsers.js

 <!-- use a module script to detect modern browsers: -->
  <script type=module>
    self.modern = true
  </script>

  <!-- now use that flag to load modern VS legacy code: -->
  <script defer>
    addEventListener('load', function () {
      var s = document.createElement('script')
      if (self.modern) {
        s.src = './module.js'
        s.type = 'module'
        s.defer = true
        s.async = true
      }
      else {
        s.src = './module.js'
        s.defer = 'defer'
        s.async = 'async'
      }
      document.head.appendChild(s)
    })
  </script>