<object type="text/html" data="urltofile.html"></object> 

3. Link Import - Not supported (due to CORS)

<link rel="import" href="/path/to/file.html">

4. Pure Javascript - WORKING (not much testing yet though)

Bold is where we add
Underline is existing HTML from GhostDoc
Intalic is content copied from TOC

Here is the treenode.js file ... 
document.write(`  
              <div class="TreeNode"> 
... all of the TOC here
              </div>
`);