复合写法：
border: 2px solid red;
分开写法：
border-color: red;
border-bottom-color: blue;
border-bottom-style: dotted;
border-bottom-width: 5px;
border-left-color: yellow;
border-left-style: solid;
border-left-width: 6px;
border-right-color: green;
border-right-style: dashed;
border-right-width: 4px;
border-style: dashed;
border-style: dotted;
border-style: solid;
border-top-color: red;
border-top-style: solid;
border-top-width: 3px;
border-width: 5px;