Is there really any excuse for allowing "<b><i>OMG!</b></i>"? No, but HTML5 is willing to pinch its nose with thumb and forefinger and look the other way. It literally is not a battle worth fighting.
Just like <b /> that causes a (perhaps several) parse error. Nothing special is done about it in HTML5.
-- Anne van Kesteren <http://annevankesteren.nl/> <http://www.opera.com/>
