Browser-contained lab instances, in the same family as the large hands-on platforms but much smaller in breadth on purpose.
The reader I have in mind is someone at the early stages of a career transition into the field, who needs exposure to the concepts before they need a sprawling catalog. Breadth is the thing being traded away, and it is a deliberate trade rather than a gap to be filled in later.
The site is a separate application on its own subdomain, so what it offers at any given moment is described there rather than here.