Still working on things, but I'll give you a treat for passing by!
You can add "includes" into html using javascript and iframes. Of course, if the connection is slow, someone has javascript disabled or for whatever reason the javascript runs slow you'll run into visual issues.
I did not make this, but I don't remember where I got it from either, but here. If you put this into an iframe (along with a source page) it'll work as an include
onload="this.insertAdjacentHTML('afterend', (this.contentDocument.body||this.contentDocument).innerHTML);this.remove()"

If you notice visual issues with the nav or badges tab on my page, now you know why!