@sgunhouse Thank you for your assistance!
It turned out that one of my plugins caused this issue... The developers should rework or patch it, because it totally destroys some sites. Thanks for your help and time!

@leocg said in Styling issue with Opera (red background and ugly rendering):

@Mordai Already performed the mandatory check with all extensions disabled and with a clean profile directory as in the instructions?

Dear @leocg,
Oh really... I forget to disable my extensions. It turned out that it is caused by this plugin: Lingvanex - Translator and Dictionary.
I don't know why, but after disabling this plugin, the styling issue dissappears.
Thank you, It seems that I found the guilty, nastly plugin... It might need a rework

Dear @sgunhouse,
The first example is a forum software website: https://www.lotus-simulator.de/index.php?user/3-janine
The second one is my personal project to test this out. This was my first time here, I noticed this issue: https://kopo.mabaomsi.ml/tagok/mabaomsi/

Best Regards,
Mordai

Dear Forum!
I noticed this issue on pretty much every website I surf on, and which has some kind "tabs" in it.
The selected tab and its content are rendered with a red, weird-looking background.
In other cases, the CSS is buggy, and the result turns into an ugly page.
Content in Chrome:


Content in Opera:

Can someone help me out, what to do? I already deleted all the cache data and cookies, nothing happened. It is interesting though that when I locate these examples in a private tab, everything works fine...

Thanks for your help!

Regards,
Mordai