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?


A web page is a combination of a structured document and a separate stylesheet. The same structured document can be styled in radically different ways just by changing the stylesheet. This is how a website can offer different skins, light/dark mode, etc.
Consider this basic HTML document:
<html> <head><title>Test Page</title></head> <body> <p>This is a paragraph.</p> <ul> <li>This is an unordered bullet-point list item</li> <li>This is also a list item</li> </ul> </body> </html>And here’s a stylesheet to make the bullet points square:
ul { list-style-type: square; }Change the stylesheet and you change how the document is rendered. “Reader mode” is just a minimalist stylesheet built into the browser.
I think OP is asking how the browser figures out what is the important content to display in reader mode and what’s the cruft that can be dropped.