It's both native and web-based. Web version: https://www.beyondloom.com/decker/tour.html

Click File -> Cards... to see how the guided tour is constructed.

Some past discussions of Decker:

4 years ago: https://news.ycombinator.com/item?id=33377964 - "Decker, a platform that builds on the legacy of Hypercard and classic macOS"

2 years ago: https://news.ycombinator.com/item?id=38985409 - "Decker – A HyperCard Replacement"

2 years ago: https://news.ycombinator.com/item?id=40292181 - "Decker: A reincarnation of HyperCard with 1-bit graphics"