Very cool concept! I was curious how it works, and yeah the source says its just details/summary HTML tags! Very cool concept. I would not have ever thought to use HTML like this. Did you use any sort of program to make this, or just write it all in a text editor?
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it was made using a tool i recently put together (originally intended for writing adventure game hints/guides): https://seansleblanc.itch.io/spoiler-friendly-hints