As long as you place it on the same domain you can do the following:
HTML
<html>
<object id="t" data="/html_template" type="text/html">
</object>
</html>
JavaScript
var t=document.querySelector("#t");
var htmlDocument= t.contentDocument;
Since the question is slightly unclear about whether it is also about elements, not just about the whole innerHTML: you can show element values that you know or guess with:
console.log(htmlDocument.data);