HttpTaskAsyncHandler and html string

[loopy159]

I am trying to implement HttpTaskAsyncHandler and returning image tag but browser is not rendering it.return string.Format("", "http://mysite/1271f1bb166275b7b8b6f2d44dd5d0d2.gif");ORreturn string.Format("< alt='' src='http://stackoverflow.com/questions/14426501/{0}' />", "http://mysite/1271f1bb166275b7b8b6f2d44dd5d0d2.gif");Chrome error: Uncaught SyntaxError: Unexpected string FF: XML can't be the whole programAny idea something changed in asp.net 4.5?Note: even I tried header content type "text/javascript" and "text/html"