CSS multi border like image

Question

Please suggest me a CSS to make border like below, many thanks

Answer 1

This is what I guess, not sure if it is the best way:

body {
  padding: 30px;
}

.container {
  position: relative;
  padding: 20px;
}

.content-container {
  text-align: center;
  border: 1px solid black;
  padding: 30px;
  position: relative;
  z-index: 2;
}

.box {
  position: absolute;
  z-index: 1;
}

.box-a {
  border-top: 1px solid black;
  border-bottom: 1px solid black;
  top: 0;
  left: 40px;
  right: 40px;
  bottom: 0;
}

.box-b {
  border-left: 1px solid black;
  border-right: 1px solid black;
  top: 40px;
  left: 0;
  right: 0;
  bottom: 40px;
}

.corner {
  width: 40px;
  height: 40px;
}

.corner-top-left {
  top: 0;
  left: 0;
  border-right: 1px solid black;
  border-bottom: 1px solid black;
}

.corner-top-right {
  top: 0;
  right: 0;
  border-left: 1px solid black;
  border-bottom: 1px solid black;
}

.corner-bottom-left {
  bottom: 0;
  left: 0;
  border-right: 1px solid black;
  border-top: 1px solid black;
}

.corner-bottom-right {
  bottom: 0;
  right: 0;
  border-left: 1px solid black;
  border-top: 1px solid black;
}

<div class="container">
  <div class="box box-a"></div>
  <div class="box box-b"></div>
  <div class="box corner corner-top-left"></div>
  <div class="box corner corner-top-right"></div>
  <div class="box corner corner-bottom-left"></div>
  <div class="box corner corner-bottom-right"></div>
  <div class="content-container">
      insert here all your content
  </div>
</div>