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.
Limitations
Publishers must only use this rel value on a post's permalink page, thereby indicating that the permalink page is a reply to the destination of the hyperlink with rel=in-reply-to
Posts that are aggregated in other pages, e.g. an archive page, or home/updates page, must not use rel=in-reply-to on links because those aggregate pages (e.g. archive, home, updates) are not replies themselves, even though they may contain replies.
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?
Nature of a “reply”
How exactly do we define “reply” so that in-reply-to is used in suitable contexts? What factors differentiate a reply from, say, a comment or mention?
- If the reply makes sense without the original content as context, is it a reply? If I write a blog post which is in response to another, but makes a point on it’s own, does it qualify to use in-reply-to? Does the content of a reply (and, by extension, multiple replies) have to flow like a conversation?
For example, a “reply” could cover almost anything from a short, two word comment (F1R5T P0ST) to a long essay, or even a video (e.g. youtube video responses). Handling all of these concurrently (e.g. trying to build a facebook/twitter-like chronological conversation UI) is likely to be a nightmare.
Previous Formats
Atom in-reply-to extension
Atom Threading Extensions (RFC4685) has an in-reply-to Extension Element.
No idea if anyone is consuming it.
Gdata in-reply-to
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
u-in-reply-to
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.
Advantage:
class=u-in-reply-tocan be used anywhere a post may occur:- post permalink page (which should also use rel=in-reply-to)
- archive pages that contain the post
- home page or recent updates that contain the post
Whereas rel=in-reply-to MUST ONLY be used on post permalink page, which is harder to get right.