The compat spec documents the way I initially implemented Mozilla's -webkit-box emulation in https://bugzilla.mozilla.org/show_bug.cgi?id=1208635 -- but that turned out to be problematic, so our impl has changed and the compat-spec text probably needs to change too.

Specifically: our impl changed in https://bugzilla.mozilla.org/show_bug.cgi?id=1257688 (see comment 0 there for the concerns that motivated the change). Unfortunately (from a speccability perspective), the resulting impl does not simply alias standard CSS properties/values (and that's intentional, due to the concerns expressed in that bug -- we don't want a page that has a crufty * { -webkit-box-pack: center } CSS rule to actually affect the modern/standard equivalent justify-content property on every single element in the page, particularly not once we've implemented justify-content for block layout. And similar for the other -webkit-box-* properties.)

Additionally, we have a few behavioral compatibility quirks that we only apply for -webkit-box and not for modern flexbox, for handling of abspos children and inline children. See https://hg.mozilla.org/mozilla-central/rev/5dcb4fe44668 and https://hg.mozilla.org/integration/autoland/rev/f42dc6b82a4d for those changes.