docs: function(parser, reporter){
          var rule = this,
              gradients;
  
          parser.addListener("startrule", function(){
              gradients = {
                  moz: 0,
                  webkit: 0,
                  ms: 0,
                  o: 0
              };
          });
  
          parser.addListener("property", function(event){
  
              if (/\-(moz|ms|o|webkit)(?:\-(?:linear|radial))\-gradient/.test(event.value)){
                  gradients[RegExp.$1] = 1;
              }
  
          });
  
          parser.addListener("endrule", function(event){
              var missing = [];
  
              if (!gradients.moz){
                  missing.push("Firefox 3.6+");
              }
  
              if (!gradients.webkit){
                  missing.push("Webkit (Safari, Chrome)");
              }
  
              if (!gradients.ms){
                  missing.push("Internet Explorer 10+");
              }
  
              if (!gradients.o){
                  missing.push("Opera 11.1+");
              }
  
              if (missing.length && missing.length < 4){            
                  reporter.warn("Missing vendor-prefixed CSS gradients for " + missing.join(", ") + ".", event.selectors[0].line, event.selectors[0].col, rule); 
              }
  
          });
  
      }