Shadow DOM 101
HTML
<div id="outer">
<div id="host">
</div>
</div>
<template id="t">
<h1>hoge</h1>
</template>
JavaScript
var d = document;
var host = d.querySelector('#host');
var shadow_root = host.createShadowRoot();
var t = d.querySelector('#t');
shadow_root.appendChild(t.content.cloneNode(true));