You could do it in this fashion below.
<head>
<link rel="import" href="https://stackoverflow.com/questions/36387676/myheadertemplate.html">
</head>
where you could have your myheadertemplate.html
<div>
<h1>This is my header</h1>
<div id = "navbar">
<div class = "Tab">Home</div>
<div class = "Tab">Contact</div>
</div>
</div>
You can then use it with JS below
var content = document.querySelector('link[rel="import"]').import;