To everybody who says ohohoho Vivaldi can't be trusted because it's proprietary...
Yes under the DEFINITION it is not Open Source because it misses ONE of the required categories to be Open Source, and that is the licensing which prevents users from customizing the UI and releasing a fork of it.
HOWEVER, the code IS there, it IS source available, and it CAN be audited.
Link for more info: https://piped.video/watch?v=oCyIzqmc_PQ&t=0