What exactly can be done with CSS? It is important to know that CSS stands for cascading style sheets. The key word is cascading. Passing information from one element on the page to another is generally restricted to passing information from a parent to its child or from a sibling to a younger sibling. In order to use CSS for interactive elements, a rather flat structure is required.
Good HTML tutorials will suggest nesting elements because HTML and CSS do a good job of placing nested elements in the correct way. However, an element in one div cannot pass information to another div. Even elements with the same parent cannot pass information back and forth because a sibling can only talk to its younger siblings.
A flat structure means that much of the work of positioning elements needs to be done manually, so you need to know the difference between relative, absolute, and fixed positions. Elements will be on top of each other so knowing how to use z-index to change the stack ordering is imperative.
Creating an interactive web page with just HTML and CSS is like a magic trick. If you see how it is done, then you will probably be disappointed by its simplicity. There is a lot of deception by placing smoke and mirrors in just the right places. If you try to figure out how things are done on your own it will be much more satisfying.
However, I am working on posts for the future that describe how the pages work. In the meantime, looking at the source may give you some clues. All the answers are in the source, but figuring out what each line does can be tricky.
This website is still a work in progress, but should be good on Firefox/Chrome on a laptop/desktop. Only the bare minimum has been done to make the site responsive so not every page really works on a phone. Safari seems to have trouble with lots of SVGs and so the House map and travelling salesman game are slow. Most things needed to be built from the ground up so any edge cases that established frameworks handle automatically could be problematic.
I hope you enjoy this website. If you think you can build similar web apps with just HTML and CSS, I encourage you to give it a shot. The process is very frustrating at times, but the result can be worth it if you enjoy doing things that other people might view as crazy. I use Hugo to generate static sites, and I suggest using something like Hugo if you are trying to create a complicated web page.
To see my sources for mapping and political data, click here. If you would like to contact me to tell me what you have built or any other reason, my email is firstname.lastname@example.org.