The first hurdle in trying to make simple multipage apps with handwritten HTML/CSS is, as the author mentioned, getting to the step where you have reused components across pages and have to jump into a server-side templating setup like PHP, or use something that introduces a build step if you want to stay static. I don't know why there isn't a simple method for including HTML partials incrementally in handwritten sites without doing either of these. WebComponents and slots seemed perfect for this but they force you to define them with JS.