“other similar links” is a vague description for what constitutes getting mentioned in HTML #91
Description
Prompted by aaronpk/Telegraph#20 and indieweb/mention-client-php#33.
I am wondering what exactly is considered a mention of a link in an HTML file. 3.2.2 Webmention Verification keeps relatively agnostic on data format but does say the following:
For example, in an [ HTML5] document, the receiver should look for
<a href="*">,<img href="*">,<video src="*">and other similar links.
(Note that this is a “for example”, using “should” within a SHOULD section of the specification.)
It calls out a, img, and video elements specifically, but does not note what type of links it considers “similar”. AFAIK there is no category within the HTML5 spec that includes all three of these elements.
HTML5 does have 4.8 Links which includes a, area, and link. Would those be considered “similar links”? An argument that came up in the IndieWeb development chat around this was that a link for Webmentions means the “*user* concept of a link”, but that would definitely exclude link elements that usually aren’t displayed.
The call-out to img and video is interesting too, as it would be a sign of hotlinking a resource, not so much mentioning it. And interesting because these use the src attribute rather than href. Supporting video is actually a multiple step thing, as it implies supporting track and source elements within the video. (Although, again, not specifically mentioned.)
Webmention Implementation Reports have a special section on HTML Verification that adds the audio element to the list of “similar links”, which like video can have track and source elements. And it explicitly bans URLs that are just part of text. This latter is not mentioned in the Webmention specification either and something implementations might not be doing.
Note that none of these tests are marked with RFC2119 key words.
Maybe every URL provided in a href or src attribute is a mention? That would cover all HTML5 Links and most Embedded content. Though for object elements you would need to check the data attribute.
Would it be helpful to extend the specification somehow to define what is and isn’t expected of Webmention receivers?
Right now it is completely up to the individual implementor to pick and choose, which could lead to some weird compatibility problems. On the other hand, it also allows IndieWeb projects to use URLs detected through microformats parsing without the need for limiting it to specific elements.
Thoughts?