Implementation note: When tangents are not specified, client implementations should calculate tangents using default MikkTSpace algorithms. For best results, the mesh triangles should also be processed using default MikkTSpace algorithms.
https://github.com/KhronosGroup/glTF/tree/master/specification/2.0
Sorry for beating a dead horse on this one. If babylonjs supports gltf, then it must also support the mikkt encoding, right?