It has nothing to do with WebGL though, that same problem might happen in native 3D engines too as soon as you need to support multiple 3D APIs from a common sh

It has nothing to do with WebGL though, that same problem might happen in native 3D engines too as soon as you need to support multiple 3D APIs from a common shading language. On the contrary, WebGL2 is probably the only 3D API that comes even close to the idea of "write once, run everywhere".