Change the page from desktop to mobile (Developer Tools)
Google Chorme and Internet Explorer have the option of switch from desktop to mobile the websites at F12, Vivaldi should have this option too, with more screen sizes, plataforms (iOS, Android and Windows Phone/10 mobile, as UC browser do), and for different types of browser.
Modedit: Added title
This feature will come in Vivaldi Developer Tools.
In a internal Vivaldi tester version the switch Desktop ↔ Mobile is implemented and working