I have something that could improve the learning process a tiny little bit:
Whenever there is a "variable" "function" or any other nomenclature or thing that it is important could be clicked to open a pop-up window.
Inside of the new pop-up window, you have a brief description for the clicked object.
It is an optional feature that would help a lot to memorize the subjects!! :D
Just like there is with lesson 15 when you talk about variable... so why not use it for the words that have color to it? :D
Something like this: