Greasy Fork

Works very well. Thank you.
However, on Reddit, some elements still have rounded corners, see the following image

Sadly, that's a current limitation of the web platform :( There will likely be more and more such places.

I've briefly described it at source of this "mirror", https://userstyles.world/style/8283/unround-everything-everywhere#shadow-dom-with-closed-root (apparently still forgetting to sync descriptions).

For reddit particularly, when desperate, I tend to still switch the URL from `www.reddit.com` to `old.reddit.com`, what mostly fits my taste better.

If opportunity arise and there will be a way to overcome this nuisance, be sure I'll implement it the moment I'll learn it :]

Anyway, thanks for the feedback, appreciated!

Alright! Don't you worry :-) Your script is very useful however!
Many thanks to you and keep up with this good work!

Hey, so turns out there is a somewhat official userscript to overcome this nuisance:

https://greasyfork.org/en/scripts/424030-stylus-shadow-dom-support.

At least Authors/moderators of Stylus point to it in GH discussion (https://github.com/openstyles/stylus/issues/739). The fact I haven't mentioned until now was that I haven't actually seen got it working in the past, but turns out I probably had "Circumvent CSP 'style-src' via adoptedSty­leSheets" in Stylus settings turned on, what broke that. Tried it now with first clean older Firefox 138 beta and Violentmokey + Stylus combo at Reddit, and it really worked even for me.
(There is a half-second delay before userCSS penetrates into the shadow DOM, what seems to be intentional, but I guess is still fine, or better than resisting elements in the page.)