NaCl is specific to individual processor architectures by design, much like ActiveX was specific to Windows.
Prefixed WebKit features are certainly not consistently well-documented. CSS animations, transforms, etc. had to be carefully reverse engineered by other browsers.
CSS animations and transforms were released along with formal specs. The Webkit developers read mailing lists, hang out on IRC, etc. If anyone thought these were underspecified or ambiguous, they could easily ask for answers or adjustments to the spec. It’s not like the Webkit folks were trying to hide anything.
You’re right. CSS animations were announced and added to Webkit betas in October 2007, and it took until the beginning of April 2008 for Apple to put up their proposed formal spec. I’m not sure when they first hit a publicly released version of Safari. Still, 6 months is pretty short in browser spec time, and pretty much no one was using these features in the wild until much later.
Prefixed WebKit features are certainly not consistently well-documented. CSS animations, transforms, etc. had to be carefully reverse engineered by other browsers.