<IFRAME ...>
LONGDESC
Similar to <IMG LONGDESC="...">, W3C describes LONGDESC as "a link to a long description of the frame". Like <IMG LONGDESC="...">, we don't know of any browsers that actually support this attribute, but it would be a worthwhile thing to support.
<IMG LONGDESC="...">
<IFRAME FRAMEBORDER="..."> >>>
<IFRAME FRAMEBORDER="...">