心血来潮 什么都不说 上代码

.gradualChange{
width: 300px;
height: 300px;
border:20px solid;
//兼容
border-image: -webkit-linear-gradient(#000,#333,#000) 30 30;
border-image: -moz-linear-gradient(#000,#333,#000) 30 30;
border-image: linear-gradient(#000,#333,#000) 30 30;
}