So far, the most cross-platform is the web, then Qt. A little apart is Firefox and XulRunner app as a browser plugin.
You can also try NodeJS -
https://habr.com/ru/post/218389/
Java of course, the leader of crossplatforming - but this is for serious people who do not find fault with the syntax.
Web, all sorts of PWA and node.js do not roll, unfortunately.
Do not roll due to the large size and voracity is very great resources. - Freddy_Konopelski commented on March 19th 20 at 08:47
So who is the leader? - Lesley.Kuhlman commented on March 19th 20 at 08:50