This is in the fixed function part of the hardware. There's no possibility for emulating the behavior. Since WebGL's hosted on top of a large range of OpenGL versions, it's impossible to guarantee OpenGL ES 2.0's behavior in WebGL 1.0 contexts.
To be honest since this hasn't come up until now I'm inclined to simply mention this in the differences section of the spec and not change the spec. Adding a query to determine the underlying behavior would only slow down startup time for most WebGL apps and add yet more code to the browser for a corner case.
-Ken