Cross-platform development approaches:

https://cordova.apache.org (https://cordova.apache.org/docs/en/latest/guide/overview/index.html)

https://software.intel.com/en-us/html5  (XDK and CrossWalk)

 

https://www.appcelerator.com

https://www.xamarin.com (https://developer.xamarin.com/guides/cross-platform/application_fundamentals/building_cross_platform_applications/part_1_-_understanding_the_xamarin_mobile_platform/)

https://www.qt.io

 

https://coronalabs.com  (example of framework mainly for gaming)

 

http://tech.co/cross-platform-mobile-app-development-2-2015-07

https://www.infoq.com/articles/mobile-cross-platform-app-development