As AES-KW can only wrap keys that are 8*n bytes bytes long, wrapping a CryptoKey in JWK format is only possible when the exported key happens to be of that size.
See <https://www.w3.org/Bugs/Public/show_bug.cgi?id=24457>. We should probably wait to see if a particular form of padding gets prescribed.
When done, we should re-add a subtest to crypto/subtle/aes-kw-wrap-unwrap-aes.html that was removed in <http://trac.webkit.org/r163244>.
Per discussion of the W3C thread and the most updated spec:
https://www.w3.org/TR/WebCryptoAPI/#aes-kw-operations, and Editor's draft:
This padding restriction becomes a check on the wrapKey operation of AES-KW. Therefore, this bug is obsolete.