Old Server Side Include method does not show in browser <!--#include file="sidebar1.html" -->
I've been updating a small business website for many years. It uses <!--#include file="sidebar1.html" --> to include a side navigation. In Dreamweaver CS3, the navigation would show up on the server but I couldn't see it in Dreamweaver. This was slightly annoying but workable. Now, updating for the first time in Dreamweaver CC, I can see the sidebar navigation in Dreamweaver WYSIWYG but it doesn't actually show up in any browser! How can I get it to work again?
I thought maybe DWCC updated something without asking so I uploaded last year's version of the site from a backup but the nav is still absent. The "sidebar1.html" file is definitely on the server. I read that this include method is very old. Did browsers decide not to support it anymore? The clients here may be shutting down after this year so they don't want to rebuild the site. I've got to fix this somehow.
I'm not good at code, but I found what may be a good replacement, so I tried using the method below, from How To Include HTML . But this also shows up in DWCC but not my browser. Can someone please help me fix or understand this?
<html>
<script src="https://www.w3schools.com/lib/w3.js"></script>
<body>
<div w3-include-html="sidebar1.html"></div>
<script>
w3.includeHTML();
</script>
</body>
</html>
