Iframe alternative content

I was somewhat surprised when reading the spec for iframes to come 
across the [content model][1] rules.

> When used in HTML documents, the allowed content model of iframe 
> elements is
> text, except that invoking the HTML fragment parsing algorithm with 
> the
> iframe element as the context element and the text contents as the 
> input
> must result in a list of nodes that are all phrasing content, with no 
> parse
> errors having occurred, with no script elements being anywhere in the 
> list
> or as descendants of elements in the list, and with all the elements 
> in the
> list (including their descendants) being themselves conforming.

This is a change from HTML 4 where it could contain %flow content which 
included block elements.

The nu validator seems happy with `<iframe><p>Test</p></iframe>` though. 
Is this is bug or am I misunderstanding the spec?


[1]: 
https://www.w3.org/TR/html5/embedded-content-0.html#iframe-content-model