They only have to distribute the source code for their modified Webkit, and IIRC have some way for end users to link in their own Webkit. That does not have the

They only have to distribute the source code for their modified Webkit, and IIRC have some way for end users to link in their own Webkit. That does not have the same vitality of a strong copyleft license like GPL. I guess this may still be classified as copyleft, but by no means does it require them to redistribute the source code of their browser. Note that Edge is not open source, even though it is based on Chromium and contains Webkit-derived code.