ONNX is a spec. ONNX Runtime is an implementation of ONNX. There are other implementations too. But ONNX is not a text spec like the RFCs for network protocols.

ONNX is a spec. ONNX Runtime is an implementation of ONNX. There are other implementations too. But ONNX is not a text spec like the RFCs for network protocols. ONNX is also a collection of C/C++ code. ONNX's implementations rely on this code to do type and shape inference. My point was: if someone wants to implement ONNX(write a library that can load and run ONNX models), he/she has to reuse this C/C++ code, or totally rewrite them in his/her favorite programming language(but I think it is not very practical). If an ONNX implementation wants to do codegen, like what XLA does, then usually it is based on LLVM and it needs to be shipped with a copy of LLVM.