I was thinking, although for acid2 Hixie decided that using -- in tags beginning <! shouldn't toggle comment mode on and off, I think it should do so in element tags. This would allow things like the following: <object foo="bar" -- this does foobar stuff -- bish="bosh" -- this hits people over the head -- bash="bush" -- this elects a democrat -- >
This has never been allowed (not even in SGML) and it isn't really possible to add now with the legacy content we have -- and in HTML5 we're not adding it. So all in all, I'd say WONTFIX.