Validator.nu accepts 

1) empty dl: <dl></dl>
2) dl with dd but without dt: <dl><dd>foo</dd></dl>

Neither of the above are legal. Content model according to the WHATWG-spec:

]]
    Content model:
       Zero or more groups each consisting of one or more
       dt elements followed by one or more dd elements,
       optionally intermixed with script-supporting elements.
[[

http://www.whatwg.org/specs/web-apps/current-work/multipage/grouping-content.html#the-dl-element

PS: This bug is not available in validator.w3.org/nu

PPS: I am not sure that the HTML5+/HTMLLiving spec is correct - e.g. may be the dt element should be allowed to be omitted from each name/value group? After all, it is possible to *fake it* by inserting an empty <dt> - something which is no different from deleting the entire dt. I might contribute if this issue is added to the bugzilla of the HTML spec(s).

PPPS: But at this moment I am not sure why <dl> should be allowed to be completely empty.