Because that thing basically describes a proprietary plugin like Activex, Silverlight or Flash before it, so a third party browser which doesn't have that proprietary tech can't fake it, under pretense of "standard". The code of that plugin will not be open source, worse, it will act as a spyware on people's computers at the OS level.
It's like EME before and these proprietary techs have no place in a open standard spec.