widget.html

<div id="my-widget"></div>
<script src="https://mydomain.com/widget.js" type="text/javascript"></script>

widget.js

(function() {
  var widget = document.getElementById("my-widget");
  widget.innerHTML = "<link type='text/css' href='https://mydomain.com/widget.css' rel='stylesheet'>" +
  "<div class='my-content'>"+
    "This is my content."+
  "</div>";
})();