Well, why is this site is mentioned here? It is because of one tiny but powerful detail: Their way of keeping user's progress. This site is all about written content and links to outer site contents curated in a way that it is like an actual curriculum. And this tiny power detail is that it has check buttons for the user to keep track and it asks for the credential after the user's first check.
This is a simple yet powerful way of keeping track of what you have learnt on the site. You don't have to manage the article tracking by yourself, site helps you to do it. Loved it! Now the question: Is this possible to generalize such a concept for all content-heavy sites?