How to create a border that fully covers the adjacent corners in CSS?

Question

I have a div with a 1px border and I\'m trying to create a 3px border in another color to that div. I\'m using this code:

box {
  border: 1px solid #ffffd;
  b         


        

Answer 1

Use css :after pseudo-class, docs

.box_big {
  border: 10px solid #ffffd;
  position:relative;
  z-index: 1;
}
.box_big:after{
    height: 10px;
    position: absolute;
    top:-10px; left:-10px; right:-10px;
    content: " ";
    z-index: 2;
    background: red;
}
.box {
  border: 1px solid #ffffd;
  position:relative;
  z-index: 1;
}
.box:after{
    height: 3px;
    position: absolute;
    top:-3px; left:-1px; right:-1px;
    content: " ";
    z-index: 2;
    background: red;
}

    big box



    your box