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

This also puts a line on top:

.box1 {
  border: 10px solid #ffffd;
  border-top: 0;
  box-shadow: 0 -30px 0 #3F9BD0;
  float: left;
  width: 300px;
  height: 300px;
}