If link tag can be inserted in body tag, and inserts importee's document.body to importer, then it would be easily without any scripts.
An example is below:
component.html:
<!DOCTYPE html>
<div>
<p class="name">Hong Gildong</p>
<p class="age">20</p>
</div>
index.html:
<!DOCTYPE html>
<html lang="en-US">
<head>
<title>Human Being</title>
</head>
<body>
<link rel="import" href="component.html">
</body>
</html>
