改变input中placeholder默认值的样式

      input::-webkit-input-placeholder{
        color:#4e4e4e;
      }
      input::-moz-placeholder{
        color:#4e4e4e;
      }
      input:-moz-placeholder{
        color:#4e4e4e;
      }
      input:-ms-input-placeholder{
        color:#4e4e4e;
      }