It seems Yahoo server does some heuristic based on User-Agent in a case Accept header is set to */*.
Accept: text/html
did the trick for me.
e.g.
wget --header="Accept: text/html" --user-agent="Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:21.0) Gecko/20100101 Firefox/21.0" http://yahoo.com
Note: if you don’t declare Accept header then wget automatically adds Accept:*/* which means give me anything you have.