rel-in-reply-to
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rel=in-reply-to is proposed method for linking from a comment to the original post that the comment is a reply to.
Publishers
In a post that is a reply/response/comment to some other original post on the web, the hyperlink from the reply to the original should be marked up with the rel value in-reply-to.
Consumers
WebMention
If your site implements receiving webmentions, implement the WebMention ping verification using rel="in-reply-to" as documented in the WebMention specification.
When your site receives a verified webmention with an "in-reply-to" link, provide a display of the webmention source on your webmention target page, e.g. as suggested in: IndieWebCamp: comment presentation (uses the h-entry at the source page to construct a comment on the target page with author, comment contents, date posted, etc.).
Issues
Replying To Multiple Original Posts
What if a post is a reply/response/comment to multiple other original posts on the web?
- Example: (need a real world example of a reply to multiple other posts to better understand this problem and its importance)
Should the reply put rel="in-reply-to" on the hyperlinks to all of the other original posts? Or should the reply only use rel="in-reply-to" on the first or most important original post that it's a reply to?
Previous Formats
YouTube uses a URL-qualified version of in-reply-to:
rel="http://gdata.youtube.com/schemas/2007#in-reply-to"
in their Atom API, e.g.:
<link rel="http://gdata.youtube.com/schemas/2007#in-reply-to"
type="application/atom+xml"
href="https://gdata.youtube.com/feeds/api/videos/2cd.../comments/978..."/>
Alternatives
Alternatively, or in addition, there is the suggestion to use:
class=u-in-reply-to
on hyperlinks from comment posts marked up with h-entry to original posts.