I don’t know if you saw it, but Vivaldi has a clock in the status bar. Some day, searching a simple calendar (Vivaldi has a full-feature calendar)… Surprise! Thereis no calendar, but a timer!
In other words, with Vivaldi (without any add-on) you can use techniques like Pomodoro!

Not only timer, Vivaldi allows to set time-based alarms. You can even configure duration and alarm sound. It’s one more reason to use this fantastic web browser!