In desktop GL/GLES2 world, extensions are prefixed with "GL_". In WebGL extension registry, all extensions start without that suffix: http://www.khronos.org/registry/webgl/extensions/ . In the function glGetString it was decided to be desirable that both unsuffixed and suffixed forms were reported, see here https://github.com/kripken/emscripten/blob/master/src/library_gl.js#L652 . Perhaps GLEW codepath should follow the suit, and be able to manage both unsuffixed and suffixed forms for consistency? (glewIsSupported and glewGetExtension)