My self-hosted website is no more showing 404 errors (of course for the pages that don't exist). I edited the 404 template of my theme and just updated the written text of the heading and body text, nothing else, and I get this now.
When I open a page that doesn't exist on my website, like (mywebsite).com/anythinguseless, the tab loads in a moment and shows just the link in the tab title. To be technical, it opens a webpage with nothing:
<html> <head></head> <body> </body> </html>