There are a few "tricks": patching the site CSSOM to apply simpler mobile styles even at desktop width, cleaning up parents of DOM text nodes, blocklists for class names that contain words like "sidebar", plus manual CSS patches for popular sites.
I got carried away and also added a dark mode, page outlines, privates notes & inline Hacker News comments. The last feature works by parsing every top-level HN comment with a quote in it (formatted with > or "") within a few minutes, and anchoring these quotes in the story article HTML. So when you open a link you'll directly see the parts people are talking about here. [1]
The extension code is all on GitHub: https://github.com/lindylearn/unclutter
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[0] Unclutter vs the Firefox reader mode: https://github.com/lindylearn/unclutter/blob/main/docs/compa...
[1] It's fun to try this on some of the "HN classics" that got 30+ quote comments over the years. The list at https://hn.lindylearn.io/best shows the number of "annotations" a link has beneath its title.