Using CSS to load HTML

Alright, so I know that technically CSS shouldn’t be used to do this but I’m not sure if it applies in this case and if it doesn’t, then I need help.

I have a subnavigation for my page that I’d like to put into a CSS file instead of having to edit it on every file that is a part of the navigation. Here’s what it looks like now:

It is on the left. I’d like to make the text links loadable from an external file but nothing I’ve tried has worked including the content tag. Any suggestions??