Extending native Custom Element

HTML

<script src="http://www.polymer-project.org/platform.js"></script>
<script src="http://www.polymer-project.org/polymer.js"></script>
<script>
    (function() {
        var XHTMLPrototype = Object.create((HTMLTemplateElement || HTMLElement).prototype);
        XHTMLPrototype.createdCallback = function() {
            console.info('created', this, this.content, this.content.children.length);
        }
        XHTMLPrototype.attachedCallback = function() {
            console.info('attached', this, this.content, this.content.children.length);
            //..
            var distributeHere = this.content.querySelector("[is='imported-content']");
            var importedContent = document.createElement("span");
            importedContent.innerHTML = "Imported content";
            distributeHere && distributeHere.parentNode.replaceChild(importedContent, distributeHere);
            if(!distributeHere){
                alert("Hey! I should have some content.");
            }
        }

        document.register('imported-template', {
            prototype: XHTMLPrototype,
            extends: "template"
        });



    })();
</script>
<ul>
    <!-- this works -->
    <template id="works" is="imported-template" bind="{{ appdata }}">
        <li>li: {{username}}
            <imported-content></imported-content>
        </li>
    </template>
</ul>
<template id="fails" bind>
    <ul>
        <template is="imported-template" bind="{{ appdata }}">
            <li>li: {{username}}
                <imported-content></imported-content>
            </li>
        </template>
    </ul>
</template>
<script>
    document.addEventListener("polymer-ready", function() {
        // global for easier debugging and fiddling
        model = {
            appdata: {
                username: "Hello"
            },
            html: "./partials/basic.html"
        };
        document.getElementById('works').model = model;
        document.getElementById('fails').model = model;
   ...

CSS

h3 {
    color:red;
}