For a long time, the standard viewport meta tag went like this - However, since iOS Safari 9.0 was introduced in September last year, a new additional value is now needed for most responsive websites, making this the new standard meta tag -

»For the last few years, the HTML5 specification has included a warning about the lack of implementations and has suggested that the document outline algorithm not be relied upon to convey heading semantics to users. Recently this has been taken a step further. Now the HTML 5.1 specification  requires developers to use h1-h6 to convey document structure. The simple reason for this change is that the HTML5 document outline is not implemented and despite efforts to get it implemented, the general response from user agent developers has not been enthusiastic. «