Currently CFPreferences Direct Mode propagation between WebKit constellation processes only supports AnyApp/CurUser/AnyHost preference types:

void AuxiliaryProcess::setPreferenceValue(const String& domain, const String& key, id value)
{
    if (domain.isEmpty()) {
        CFPreferencesSetValue(key.createCFString().get(), (__bridge CFPropertyListRef)value, kCFPreferencesAnyApplication, kCFPreferencesCurrentUser, kCFPreferencesAnyHost);
#if ASSERT_ENABLED
        id valueAfterSetting = [[NSUserDefaults standardUserDefaults] objectForKey:key];
        ASSERT(valueAfterSetting == value || [valueAfterSetting isEqual:value] || key == "AppleLanguages");
#endif
    } else
        CFPreferencesSetValue(key.createCFString().get(), (__bridge CFPropertyListRef)value, domain.createCFString().get(), kCFPreferencesCurrentUser, kCFPreferencesAnyHost);
}

We should support propagation of other preference types too, for example CurrentHost.