设置input 中placeholder的样式

::-webkit-input-placeholder { /* Chrome/Opera/Safari */ 
  position: relative;
  top: 4px;
}
::-moz-placeholder { /* Firefox 19+ */ 
  position: relative;
  top: 4px;
}
:-ms-input-placeholder { /* IE 10+ */ 
  position: relative;
  top: 4px;
}
:-moz-placeholder { /* Firefox 18- */ 
  position: relative;
  top: 4px;
}