It isn't in the HTML specification. It is a W3C Working Draft.
W3C isn't just HTML: they also publish the specifications for CSS, ARIA, XML, XSLT, XQuery, XProc, RDF, SVG, WOFF and so on. Some of which (like the XML-related stuff) have no real bearing on the browser or front-end tech.
I agree. I feel more and more sorry for browser architects every day. It seems now more than ever that modern browsers are just kitchen sinks for spec writers. I'm fully aware some of these spec authors are browser architects themselves, but cut the fat please.
We'll be stuck with these redundant proposals for decades. Webmentions could be a fantastic library on its own, though as previously mentioned, we already have trackbacks. Does it need to be an additional few files in the codebases of Chromium, Firefox, et al.? No.
Where does this spec have anything that requires code in a browser? It's purely a server-to-server protocol, to be implemented by blog engines, CMS or third party services like disqus, ... (And has nothing to do with "the HTML specification" either)
You're missing the point of standardization - it's not a legislative process that requires implementation, it's a documentation process to show what has already been implemented and how you can interoperate with it.
Webmention already has multiple libraries and services implementing it.
And in cases like this, standardization is just about the only way to ensure the feature's success. If every blog/publishing engine developed their own proprietary service for tracking linkbacks/mentions, it would likely never get off the ground at all.
