It's Bootstrap, innit.
These pages were written by hand as a learning exercise.
This page follows along with the sections of the Tutorial Republic Bootstrap tutorial
There is a meta element <meta name="viewport" content="width=device-width, initial-scale=1"> in the page header. This tells the browser to not attempt to scale down the page, but instead to use media queries to figure out layout on narrow screens such as phones etc. Note that this is not part of any web specification - it is just a widely supported convention (originated with Apple). The UX aim is to avoid horizontal scrolling. The content values themselves mean the following:
width=device-width special value meaning the viewport is the width of the device screen
initial-scale=1 the initial zoom level of the page (i.e. 100% - one CSS pixel is the same size as one device pixel)
Some useful links about Viewport:
W3S Schools
Quirks Mode
Web Code Geeks
Mozilla Developer Network
In the header there is a link to the Bootstrap CSS via a CDN. Instantly makes the page look better!
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap@4.6.0/dist/css/bootstrap.min.css">
At the foot of the body there are links out to JS scripts for Bootstrap and JQuery (which is a dependency)
<script src="https://code.jquery.com/jquery-3.5.1.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/bootstrap@4.6.0/dist/js/bootstrap.bundle.min.js"></script>