Are you seriously suggesting that the primary motivation for this feature to be added to CSS is to make life harder for new browser engines?

The thought had crossed my mind, yes. Do you think Google does anything from the kindness of their hearts, without considering Chrome's position? It's one of their most valuable assets.

It's needlessly conspiratorial to ascribe intention here, especially when the Servo community have already expressed interest in supporting upcoming (difficult!) standards like CSSOM