It's completely and utterly irrelevant that Chromium is open source, because the web is a protocol, and having the source for an implementation of the protocol doesn't matter in the least when you don't control the protocol. You can't just fork Chromium and remove a feature, because websites expect the feature, and your browser won't work on them. You can't just fork Chromium and add a feature, because websites don't care about your tiny fork and won't use your feature. You can't fork Chromium, you have to fork the entire web.
That's exactly what we need to do. More specifically, we need to decouple the app web from the document web. Most of the value of the web to society lies in text, images, and video, in that order. We need a version of the web refocused around basic content with a spec simple enough for a small team to implement a browser for. A subset of HTML/CSS is probably the only way to succeed, since sites would need to work with current browsers. I think a few HTML tags + flexbox + fonts + colors would get you pretty far.