Any graphics operations directly involving OpenGL need to be executed on the rendering thread. Doing so on a different thread results in undefined behaviour. This is due to the OpenGL context only being active on the rendering thread. Making the context current on another thread has its problems on a lot of Android devices, hence it is unsupported.

No class in libGDX is thread-safe unless explicitly marked as thread-safe in the class documentation.

HTML5

JavaScript is inherently single-threaded. As such, threading is impossible. might be an option in the future, however, data is passed via message passing between thread. Java uses different threading primitives and mechanisms, porting threading code to Web Workers will not be straight forward.