borderの正方形の中にバツを付けるサンプル

#s {
  position: relative;
  width: 100px;
  height: 100px;
  border: 1px solid #C00;
}
#s::before,
#s::after {
  position: absolute;
  top: 50%;
  left: 0;
  content: "";
  display: block;
  width: 100px;
  border-top: 1px solid #C00;
}

#s::before {
  transform: skewY(-45deg);
}

#s::after {
  transform: skewY(45deg);
}