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Re:Vivaldi – A new browser for our friends by quarterto on Tuesday January 27, 2015 @04:27AM (#8952136)
Since it seems to use atom-shell for the UI[1], it's likely Chromium.

[1]: https://vivaldi.com/#Features/4

Re:Vivaldi – A new browser for our friends by Maakuth on Tuesday January 27, 2015 @06:00AM (#8952424)
User agent provides further evidence, mine reports to be Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/40.0.2214.89 Vivaldi/1.0.83.38 Safari/537.36