Specifically, Microsoft says that the GLSL::HLSL transpiler portion of the renderer is being offered to the open source community. It added:
WebGL web pages run 'programs' (shaders) on your GPU to render 3D content; these programs are written in the GLSL shading language. Microsoft Edge uses the DirectX subsystem in Windows (and HLSL, a different Windows-specific shading language) to render content. Microsoft Edge converts WebGL content to DirectX equivalents to display it; the WebGL renderer converts WebGL calls into DirectX equivalents, and the transpiler converts GLSL shaders to HLSL shaders.