XAML, but it Microsoft-specific. While not completely uniform its use does extend across desktop and mobile. I wouldn't make the argument that it's intuitive or simple, but it is powerful.

WPF is open source[1], but there isn't any effort to port it as far as I know. WPF also had a few warts compared to Silverlight.

[1]: https://github.com/dotnet/wpf/

JavaFX and QML are pretty much based on XAML.

Xaml and Javafx where released within 6 months of each other I kinda expect parallel implementation rather than copying. Just form the dates in Wikipedia

JavaFX was initially J3, and then it was rebooted into XAML like.

Also you're forgetting Longhorn SDK was available as preview before the project was canned.

But yeah just conspiracy theories from my side.

MXML. The Markup system for Adobe Flex is almost certainly the inspiration for both.

React Native? SwiftUI?

Lazarus/Freepascal. Cross platform, real native x86_64/ARM/Atmel/Etc.

GTK sort-of fits the bill

I recently built something with PyQt bindings and it works really well:

https://gitlab.com/djsumdog/mpvbuddy

In my previous job I wrote a couple of not-too-complicated apps targeting Linux in PyGTK and I found it to be pretty pleasant, except when it came to custom styling.... mainly because the styling language does didn’t seem well documented and i found it to be somewhat arcane. Didn’t stop me though and the project got done. I wouldn’t mind working with it again

There's JavaFX with FXML.