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Google's Web DRM is Worse than I Thought...
(www.youtube.com)
This is a most excellent place for technology news and articles.
Although Chromium/Blink is forked from WebKit, it's far from being WebKit these days.
But of course, Vivaldi could base their browser on WebKit or Gecko. Many of these "smaller" browsers tend to be based on Chromium though, likely because it's the most compatible (because of its marketshare).
And it's likely too much work for them to switch engines now.