I tried googling and I got a lot of hits for what reader mode does and how to enable it, but nobody can explain what the browser is actually doing.
Do sites have pared-down versions of their pages specifically for reader mode? Or does the browser just scan for particular HTML tags?
I don’t have any insider knowledge, but I’d guess they’re basically just rendering the html without JS or the original CSS, probably with a few tweaks. They may also look for certain tags (like “content”) to prioritize the main section. Web developers have a vested interest in notating the content for SEO reasons.