It's really nice that browsers offer reader modes, but they are frustratingly incomplete.
Really, let's switch to a user perspective once and consider - what if I always want reader mode? This is technologically complete impossible and all the solutions are a band-aid.
Firefox and others' attempts rely on the page authors' goodwill. But some pages will always attempt to frustrate reader modes.
Alternative approaches for content extraction use machine learning such as [1], but they of course need to be updated for culture- language- and technology-specific changes.
It's a mess and will remain so for the foreseeable future.