The fact that this shader doesn't write gl_Position has been raised as an issue before, but it is a spec violation for a link error to occur in response. See section 10.13 "gl_Position" in the GLSL ES 1.0.17 specification. If a particular driver is failing to compile the shader, then the shader translator will need to be modified to automatically inject a dummy write of gl_Position, as you've done here. This change shouldn't be made to the conformance test.