detect css @Viewport via Modernizr

atviewport.js

  Modernizr.addTest('atviewport', function() {                                   

    return Modernizr.testStyles('', function(node, rule) {
      var style = document.getElementById('smodernizr');                         
      var sheet = style.sheet || style.styleSheet;  
      var supportsAtViewport                                                   

      for (var i = 0; i < Modernizr._prefixes.length; i++) {                                
        try {                                                                    
          var viewport = sheet.insertRule('@' + prefixes[i] + 'viewport{}', sheet.rules.length - 1);
          viewport = sheet.cssRules.item(viewport);                          
          supportsAtViewport = viewport.type == 15;                                
        } catch (e) {}                                                            
      }                                                                          

      return supportsAtViewport;                                   

    });                                                                          
  });