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A few others are CSS Grid Garden [0] and Flexbox Froggy [1], both multi-level games that teach CSS grid and flexbox techniques with one new concept per level. There's also Flexbox Zombies [2] which requires signup but is still fun and free.

Agreed. Went through it twice and that combined with a bit of real-world practice made me pretty proficient at RegEx.

Fantastic site.

It's really nice and well thought out.

I am curious who did these? Why would they choose to remain anonymous?

SqlBolt is fantastic. I got an entire team up to speed in sql by having them go through it.