The following code works in chrome allowing declarative shadow dom parsed and inserted with javascript. 

<script>
  const html = `
    <div>
      <template shadowrootmode="open"></template>
    </div>
  `;
  const div = document.createElement('div');
  div.innerHTML = html; // No shadow root here
  const fragment = new DOMParser().parseFromString(html, 'text/html', {
    includeShadowRoots: true
  }); // Shadow root here
</script>

However includeShadowRoots parameter is ignored by WebKit

For more info:

https://developer.chrome.com/articles/declarative-shadow-dom/#parser-only