Just finished implementing support for receiving Private Webmentions in https://webmention.io and my website! If you're using webmention.io, you will notice a new "private" property when you list mentions or get callbacks.
When you send a private webmention for a comment on one of my posts, you'll see the comment counter increment but you won't see the comment appear. When I log in, I will see the comment along with a little "lock" icon so that I know it's a private post.
WeChat ID
aaronpk_tv
