@bw well done! I teach computer science to humanities students and as much as I can tell, this is really accessible for them. In particular, I was curious how you introduce files, hosting and content reuse, and you've done a great job to keep all this simple.
@bw one doubt I have is when you introduce relative paths. I'm not saying I know better, it's my own doubt, but I'd like to hear your comment on it if possible. Wouldn't you think that a broader explanation (an aside?) is due where you write:
"We can fix this by updating the image’s src value. Add the folder name and a slash before the filename."
and few chapters further
"So, how do we refer to files that aren’t in the current folder? We can use the .. syntax like this."