As I understand it, WebKit's feature flags will not include the webkit prefix:

"WebKit’s new policy is to implement experimental features unprefixed"

But Mozilla still plans for experimental features to be prefixed:

"Mozilla supports a number of extensions to CSS that are prefixed with -moz-.

Some of these properties have been included in draft CSS specification for inclusion in the final recommendation, but are still experimental... Once standardized, and the support for the un-prefixed variant added, the prefixed properties are dropped."