gistfile1.html

/*** Option 1 ***/

<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 ***/

<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>