There's this which I'm hopeful for: Android Components [1]. If Mozilla make it easy for Android apps to use Servo as the shell for modern web apps, then the improved performance might make it worth it to "restrict" yourself to using only the features supported by Servo (i.e. without many of the workarounds that make old websites work).

[1] https://github.com/mozilla-mobile/android-components/

There are enough similarities that Igalia (https://www.igalia.com/) could implement CSS Grid for Blink and WebKit at about the same time.

They share less and less. Blink did a `rm -rf third_party/Webkit` recently.

Patches is not the only mechanism to keep code in sync.

For two engines forked from the same source (same architecture and even same code tree structure) it is relatively easy to copy/paste feature implementation, no?