com-wordpress:

[1] Topic: RSS Feed and NameSpace « WordPress.org Forums ( (2017-03-23 13:03:02 +09:00)) <https://wordpress.org/support/topic/rss-feed-and-namespace/>

line 28, column 0: Use of unknown namespace: com-wordpress:feed-additions:1 (11 occurrences)

[2] Reader Integration: Add ids to RSS feed · Issue #996 · Automattic/jetpack ( (2017-03-23 13:03:12 +09:00)) <https://github.com/Automattic/jetpack/issues/996>

Yup. Was thinking something like <site xmlns="com-wordpress:feed-additions:1">1234</site> inside the rss/channel element for an RSS feed and the feed element of an Atom feed. The namespace here is something I just made up, so feel free to use something more sane.

[3] How To Use RSS Feed In WordPress (What Is RSS) « Global Blurb ( (2017-03-23 13:03:53 +09:00)) <http://globalblurb.com/use-rss-feed-in-wordpress-what-is-rss/>

<site xmlns="com-wordpress:feed-additions:1">105219076</site> <item>

[4] (2017-12-12 20:18:14 +09:00) <http://www.soho-tokyo.com/feed>

<post-id xmlns="com-wordpress:feed-additions:1">45686</post-id> </item>