Facebook first needs to bring HHVM up to full compatibility before they try altering the spec. I also discovered Facebook makes all their "contributors" (anyone who does a pull request) sign an agreement with them, which seems weird and not cool. https://code.facebook.com/cla
Defining a spec (there isn't a pre-existing spec that is being "altered") is an important part of moving to acheive compatibility, since its how you know what the target is.
HHVM spawning a spec effort for PHP is a lot like Rubinius spawning RubySpec.