Before any code, the ground it stands on: what the frontend is, what a browser does between the moment you press Enter and the moment you see a page, who decides what browsers support, the two tools you need, and a first page of your own.
A web page looks like a single thing, but it is made of three different languages doing three different jobs, and a browser that does a surprising amount of work to put them together. Knowing that shape before writing any code is what makes the rest of the book make sense: why a missing file breaks the look of a page, why a script can freeze it, why the page you see is not quite the file you wrote.
Five topics. What the frontend is and what each of the three languages is for. What happens between the address bar and the pixels on your screen. The three browser engines, the public standards they follow, and the two references that tell you what is safe to use. Setting up an editor, a preview server and a browser. And your first page: a folder, one file, about ten lines, and the loop of editing, saving and looking that the rest of the book repeats.