<div class="inset-border">文字内容</div>
body {
  background: darkcyan;
  padding: 30px;
}
.inset-border {
  padding: 40px;
  color: #fff;
  background: linear-gradient(darkred, darkred) no-repeat center / calc(100% - 30px) 100%,
    linear-gradient(darkred, darkred) no-repeat center / 100% calc(100% - 30px);
  filter: drop-shadow(0 4px) drop-shadow(4px 0) drop-shadow(0 -4px) drop-shadow(-4px 0);
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.