https://adobe-flash.github.io/crossbridge/
Before asm2js was even an idea, Unreal compiled to Flash via CrossBridge.
https://www.youtube.com/watch?v=xzyCTt5KLKU
https://www.youtube.com/watch?v=ZmZS7B1pLWg
To this day, with exception of maybe PlayCanvas, there are hardly any tools that can match the same developer experience for WebGL, almost 15 years later.