My apologies if you found that disorienting! You are right that CS101 becomes a prerequisite for CS253 (if you don't know python already). The reason I started with CS253 is because, personally for me, it helped a lot to know the top level concepts of "How web works" and the front-end (because that is what a normal user sees) and then go into concepts of server side languages which might seem too involved and dry to a beginner.