Windows is likely a big task for the same reasons as SMAP (
https://github.com/microsoft/MSRC-Security-Research/blob/mas...). XNU should be comparable to FreeBSD, which CheriBSD is a fork of, as both use Mach's VM for memory management and have a bunch of shared code in various places, but userspace is more of an unknown quite how much effort it'd be (you'll need to port Objective-C and, now, Swift, for example). For Chromium we have ported WebKit, so I'd imagine Blink isn't too dissimilar. V8 is likely interesting, though we have a version of WebKit's JSC JIT for Morello, which gives confidence in V8 being doable.