It is certainly the closest we have (and am a big fan) but I don't believe it is standardized by POSIX.
Yes POSIX was quite possibly formalized before X11 but it has had many years to include it and no standards body really has.
It seems pixel based drawing has a number of complexities that cannot be standardized; I cannot fathom a reason why. Lack of skill or leadership in the IT industry perhaps?
And now Wayland is going to distract people for ~5 years until it gets dropped and something decent gets written as a real replacement. Meaning a standard display system is going to be delayed for another decade at least.
Oddly enough I think CDE was a (now widthdrawn) POSIX standard. However looking around at the entire industry... we are no longer as capable as we once were ~20 years ago. I don't think we can come up with meaningful standards anymore. Too many broken companies with terrible short-sighted monetization ideas rather than actual progress. Newer really isn't better these days.