四角为直角样式

<div class="zhiJiaoBox">
    <div class="zhiJiao">四角直角</div>
</div>

 1 .zhiJiaoBox{
 2     width: 400px;
 3     height: 400px;
 4     background: #ccc;
 5     display: flex;
 6     justify-content: center;
 7     align-items: center;
 8 }
 9 .zhiJiao{
10     width: 200px;
11     height: 200px;
12     display: flex;
13     justify-content: center;
14     align-items: center;
15     /* 核心代码 */
16     border-image-source: radial-gradient(60% 60%, transparent 0px, transparent 100%, #999 100%);
17     border-image-slice: 1;
18     border-width: 1px;
19     border-style: solid;
20     border-image-outset: 1cm;
21     /* 核心代码 end */
22 }