Just install a browser plugin that allows you to load a custom stylesheet on every page.
Unfortunately I'm seeing an increasing number of websites using 500 <div>s with 45 nested layers of <div>s within each <div> to create a table instead of <table>s so it wouldn't work on those.
My take is that default styling is already bloated enough. Every-time I need to start out a new project, the first thing I do os to reset default styles.
Also, we already have a situation where different browsers have different defaults leading to an overall worse UX of the web.
Note that this requires giving the plugin permissions to read and write to all pages you visit. Browser extensions are routinely compromised by bad actors to steal banking passwords or bitcoin wallet keys.
I use tampermonkey, but I have it restricted to just one domain, which it complains about every time chrome is restarted.
This definitely seems like the sort of change that would break things in unexpected ways. I think the main reason is probably that a lot of websites used tables to do grid layouts before CSS grid was a thing, so having sticky headings would break this. It's really a huge failure of the semantics of HTML that a table is viewed as "an arbitrary grid of elements" rather than "a way to display tabular data".
Back when we'd use HTML tables for the entire layout. CSS wasn't a big thing and it couldn't do what you'd want. Even today plenty of emails are done via table hacking. I think your overall point is correct but the origin of the behavior, both human and web layout, are older than that.
Unfortunately I'm seeing an increasing number of websites using 500 <div>s with 45 nested layers of <div>s within each <div> to create a table instead of <table>s so it wouldn't work on those.