gistfile1.js

// create a <test-span> element
var myElement = document.registerElement('test-span', {
    prototype: Object.create(HTMLSpanElement.prototype),
    extends: 'span'
});

// extend <test-span> to <test-span is="test-span-extended">
var myExtendedElement = document.registerElement('test-span-extended', {
    prototype: Object.create(myElement.prototype),
    extends: 'test-span'
});