border left stackoverflow
* { box-sizing: border-box; }
html, body { height: 100%; margin: 0; }
#box4 {
width: 100%;
height: 80%;
background-color: gray;
float: left;
position: relative;
}
#leftSide, #rightSide {
height: 100%;
}
#leftSide { width: 50%;}
#rightSide { left: 50%; width: 50%; border-left: 1px solid #cdd0d4;}
#leftSide, #rightSide { position:absolute; }