改變input中placeholder默認值的樣式

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