There are some good criticisms here about some of the discontinuities in difficulty, but first and foremost I really want to congratulate OP for doing something

There are some good criticisms here about some of the discontinuities in difficulty, but first and foremost I really want to congratulate OP for doing something to address the instant gratification problem in learning to program. You can open this site in your browser and immediately start interacting with code rather than trying to get your java environment/editor/etc. set up, and that's fantastic, we really need more tools like this.