I guess nobody tried before. What does it require?

That's the problem - the requirements aren't spelled out anywhere. I didn't realize that at the time I asked the question. There seems to be some algorithm that tries to figure out what sites are compatible. The most info I've found is here: https://stackoverflow.com/questions/30661650/how-does-firefox-reader-view-operate

In theory this is the algorithm: https://github.com/mozilla/readability

I've tried changing my template and such, but can't quite figure it out. Here's my site: https://blog.nundrum.net/

:man-shrugging:

Hah yeah 😅 Thanks for entertaining the idea. If I figure out anything, I'll let you know.