gistfile1.html

/*** Option 1 - no ES6, all using defaults ***/

<element tagname="x-foo">
  <script>
    // By default, the tag name is used to generate the
    // named constructor object - without rebinding 'this'
    XFooElement.prototype.bar = ...
  </script>
</element>


/*** Option 2 - no ES6, using optional attributes ***/

<element tagname="x-foo" extends="SVGElement" constructor="SuperVG">
  <script>
    // By choice, the developer can extend from an existing
    // tag using extends="", and/or specify a non-default constructor
    // name via constructor="" - again, without rebinding 'this'
    SuperVG.prototype.bar = ...
  </script>
</element>

/*** Option 3 - dev-optional use of ES6 ***/

<element tagname="x-foo" constructor="SuperVG">
  <script>
    // By choice, the developer can specify a non-default constructor
    // name via constructor="", then extend that using ES6.
    class SuperVG extends SVGElement {}
  </script>
</element>