904
Google's New Web Environment Integrity Proposal Dismissed by Brave, Mozilla, and Vivaldi
(news.itsfoss.com)
This is a most excellent place for technology news and articles.
If only Google supports this, the rest can fork it together.
Forking would be insane because of how much code is in Chromium and how much work goes into maintaining it. The realistic thing to do is to keep doing what they've been doing: maintain a modified branch. WEI would just be one of many changes between Google's version and other vendors'.
So: a fork. It's not unusual for a fork to regularly merge back the upstream changes while maintaining its own set of changes.