Unfortunately, DisplayPort and HDMI specifications are kept private unless you're a paying member. I've successfully implemented DisplayPort 1.2 in an FPGA from specification documents I found, but I could never find the specification for anything better.
My understanding is that display-port is quite open in that there is no per-device fee for implementing it, I suspect you still have to be a vesa "member" to get a legit copy of the spec.
Honestly at this point I consider VESA one of the good guys. At least compared to the alternative, spits, HDMI