I got a html file like this,
//some content
//some content
You can use element with rel attribute set to import, href set to path to file, type set to "text/html". Use XMLSerializer() instance .serializeToString() with replacement document .doctype property as parameter to get declaration from imported document; document.write() with .import.documentElement.outerHTML property of link element as parameter to replace existing .doctype and node with .doctype and of imported document.
//original content