One of the ways that the Yadis specification allows for the XRDS document location to be declared is via the X-XRDS-Location header embedded via a
<meta http-equiv="..."> element in an HTML document.
HTML 5 has constrained
<meta http-equiv="..."> so that it only supports a selection of explicitly allowed headers. At the time of writing, these are "Content-Type", "default-style" and "refresh".
The result of this is that it is impossible to use Yadis in this way while having a conforming HTML 5 document. The current ethos for HTML 5 seems to be to remove any mechanism by which it can be extended in any way without going through the HTML working group and changing the core spec. While I can see the arguments for this in many cases, I don't really see the harm in allowing arbitrary extension HTTP headers (that is, those with an "X-" prefix) to be used in this way when there is a third-party specification that allows it.
It could be argued, however, that Yadis isn't using the
http-equiv mechanism in the way it was intended to be used even in HTML 4. The HTML 4 spec for
<meta http-equiv="..."> describes it as a mechanism used only on the server; the intention, I guess, was that HTTP servers would parse the HTML before it was served and create "real" HTTP headers matching the values given. I've never seen an HTTP server that actually does this, but it was apparently never intended to be processed by clients.
Fortunately, those who wish to use Yadis while still having a conforming HTML 5 document can use the proper HTTP header X-XRDS-Location. The same cannot be said for OpenID's extensions via
<link rel="...">, which are apparently not allowed in HTML 5 either.