HandheldFriendly 메타태그의 정의를 몰라서 찾아봤는데 스택오버플로우 잘 정리되어 기재있어서 가져왔다.

This tag was originally used to identify mobile content in AvantGo browsers, but became a general standard for identifying mobile websites. However, it’s unknown what range of browsers support this meta tag:

출처 : http://stackoverflow.com/questions/1988499/meta-tags-for-mobile-should- they-be-used