Whitelist <dfn> element
Bug 671. Harmless, why not. The remaining elements in this vein that
we might want to whitelist include
* <q>: Not yet, inconsistent quote mark handling in browsers
* <address>: Semantics in HTML5 appear to mean it must represent contact
info for the author of the <body> (or <article> if present), which is
almost certain to be useless in user-submitted wiki content; see
http://www.w3.org/Bugs/Public/show_bug.cgi?id=10255
* <kbd>, <samp>: Will add if this is WONTFIXed:
http://www.w3.org/Bugs/Public/show_bug.cgi?id=9919