缺角边框

<div class="out-rect">
<div class="in-rect"></div>
</div>

.out-rect {
margin-top: 30px;
display: flex;
align-items: center;
justify-content: center;
width: 200px;
height: 80px;
padding: 5px;
background: linear-gradient(-45deg, transparent 9px, #58a 0) top right;
background-size: 100% 100%;
background-repeat: no-repeat;
}
.in-rect{
width: 100%;
height: 100%;
background: linear-gradient(-45deg, transparent 8px, #fff 0) top right;
background-size: 100% 100%;
background-repeat: no-repeat;
}