Medium.com Does Not Work
Vivaldi browser does not let you write blogs on Medium.com. Not an approved browser, however, Chrome is on there. Have to go back to Chrome to write to the blog.
But you can change the user-agent with extensions like User-Agent Switcher for Chrome
Just added User-Agent Switcher for Chrome, as suggested by Xirzag, but only after creating a new user-agent, with the real Chrome string, that can be captured in this site (I added to the Chrome group and called it NoVivaldi), that Medium worked with the Chrome user-agent.
Firefox also worked fine without any changes, but I prefer the browser identifies itself as Chrome (for developing purposes). I didn't test with other browsers.