Creating a Static/Fixed Nav Bar with Only HTML - html

Question

hope you're doing well.
My boss recently asked me to rebuild the nav bar on our site.
He wants to move away from our current method of just having the nav code on each and every web page - so we can make edits to the nav bar periodically that will be automatically implemented across the website.
He built our website back in the early days of web dev, and as such it is basically an HTML animal that really just gets the job done (and has for years) but is not responsive or pretty.
He's adamantly against using any sort of PHP or JS, since my initial thought was a JQuery script to grab a nav.html file on each page. Bummer!
That being said, does anyone know how I can essentially create the same results? Having a singular file or block of code that can be changed and then reflected on all pages across our site , using only HTML?
Thanks in advance everyone!
Here is the code we currently have copied to all of our pages, which was created long ago:
<div id="navbox">
<!----BEGIN NAV----->
<!-- #BeginLibraryItem "/library/mainsite_nav.lbi" -->
<link href="css_files/nav_control_main2.css" rel="stylesheet" type="text/css" />
<ul id="nav_horiz">
<li class="navbar"></li>
<li class="navbar">Products
<ul>
<li class="navdrop">Underwater Lasers
<ul class="flyout">
<li class="navdrop">Underwater Scanners</li>
<li class="navdrop">Subsea Scanners</li>
<li class="navdrop">Nuclear Underwater Scanner</li>
<li class="navdrop">Laser Scanner PanTilt Arm</li>
<li class="navdrop">Angle/Distance Scanner</li>
<li class="navdrop"> </li>
</ul>
</li>
<li class="navdrop">Nuclear Inspection
<ul class="flyout1">
<li class="navdrop">Core Verification System</li>
<li class="navdrop">Nuclear Underwater Scanner</li>
<li class="navdrop">Inspector In-Tank Robot</li>
<li class="navdrop">Upper RPV Measuring Robot</li>
<li class="navdrop">Restricted Area Access Control</li>
<li class="navdrop"> </li>
</ul>
</li>
<li class="navdrop">Robotics<a href="robot_inspector_land.html">
<ul class="flyout2">
<li class="navdrop">Inspector In-Tank Robot</li>
<li class="navdrop">Surveyor Inspection Robot</li>
<li class="navdrop">Press Die Vision System</li>
<li class="navdrop">Upper RPV Measuring Robot</li>
<li class="navdrop"> </li>
</ul>
</li>
<li class="navdrop">Machine Vision
<ul class="flyout3">
<li class="navdrop">Line & Area Scanning</li>
<li class="navdrop">Non-Contact Gauging</li>
<li class="navdrop">Surface Defects</li>
<li class="navdrop">Pattern Matching</li>
<li class="navdrop">Assembly Verification</li>
<li class="navdrop"></li>
<li class="navdrop">FME Console</li>
<li class="navdrop">QuadCam Zoned Surveillance</li>
<li class="navdrop"> </li>
</ul>
</li>
<li class="navdrop"> </li>
</ul>
</li>
<li class="navbar">Industries
<ul>
<li class="navdrop">Nuclear Power</li>
<li class="navdrop">Oil and Gas</li>
<li class="navdrop">Marine</li>
<li class="navdrop">Automotive</li>
<li class="navdrop">Bottling & Canning</li>
<li class="navdrop">Aerospace</li>
<li class="navdrop">Other Industries</li>
<li class="navdrop"> </li>
</ul>
</li>
<li class="navbar3">Technologies
<ul>
<li class="navdrop">Underwater Laser Scanning</li>
<li class="navdrop">Nuclear Core Validation</li>
<li class="navdrop">Inspector In-Tank Robot</li>
<li class="navdrop">Line & Area Scanning</li>
<li class="navdrop">Non-Contact Gauging</li>
<li class="navdrop">Surface Defects</li>
<li class="navdrop">Pattern Matching</li>
<li class="navdrop">Assembly Verification</li>
<li class="navdrop">Robotics<a href="robot_inspector_land.html">
<li class="navdrop"> </li>
</ul>
</li>
<li class="navbar">Newsroom</li>
<li class="navbar">About Us
<ul>
<li class="navdrop">Core Competence</li>
<li class="navdrop">Company Background</li>
<li class="navdrop">Academic Website</li>
<li class="navdrop">Careers at Newton</li>
<li class="navdrop"> </li>
</ul>
</li>
<li class="navdrop">Contact Us</li>
</ul><!-- #EndLibraryItem -->
</div>