Not open source, but apparently not fully closed source either: https://vivaldi.com/source/.

As would I, but where exactly are we going comrade?

I thought this was a really fascinating video, since Apple Explained is one of the biggest Apple-Stans on youtube. He finally realized that his favorite company is wrought with greed, and was willing enough to make a video about it. The content or his ideas don’t necessarily matter; it’s his revelation that’s intriguing.

Imagine a completely different OS running software made for your OS better than your actual OS could. This is Microsoft Windows