Microsoft Flight Simulator uses a platform called Coherent GT. This platform lets developers create video game user interfaces using HTML, CSS, and JavaScript.
Version
The most recent version of Coherent GT came out in 2016. It includes the 2016 version of the Chrome browser, specifically version 49.0.2623. You can review the compatibility differences between 2016 and today on Can I use.
The User Agent of the sim's UI is "Mozilla/5.0 (Windows NT 6.2; Win64; x64) AppleWebKit/604.1.38 (KHTML, like Gecko) Chrome/49.0.2623 Safari/604.1.38 CoherentGT/2.0"
What doesn't work
Keep in mind that the following features aren't supported in MSFS specifically:
- Audio
- Video (except WebM)
- WebGL
Debugging
The Flow code editor will give you a console for basic debugging purposes.
To debug your scripts in more detail, navigate to http://127.0.0.1:19999 in your browser or use the Coherent GT Debugger executable that comes with the MSFS SDK: SDK\Tools\CoherentGT Debugger\Debugger.exe and select //42 Flow from the list.