I don't think there are any remotely mainstream platforms there isn't an x11 server for.

XQuartz does not properly support HiDPI, so it's never going to be great on Macs anymore:

https://github.com/XQuartz/XQuartz/issues/91

Besides that, even Linux is slowly moving to Wayland.

Well, there is a Wayland Compositor for macOS:

https://github.com/owl-compositor/owl

It still lacks a lot of features though (I think, I never tried it out)

Moving to Wayland does not prevent X11 apps. And I don't think anyone would expect this to be a good solution for crossplatform guis. More of a fun starting point.

Worst case, there's always

http://www.jcraft.com/weirdx/

There's also xpra, which gets you a browser interface to X apps.

Good point. Blink could even ship with a X11 server. (Or Wayland compositor? To be more modern)