How to draw diagonal lines with css

You can achieve the desired effect by using just one single div. Check the DEMO.

div{
  border:1px solid gray;
  width:28px;
  height:28px;
  position:relative;
}

div:after{
  content:"";
  position:absolute;
  border-top:1px solid red;
  width:40px;
  transform: rotate(45deg);
  transform-origin: 0% 0%;
}

Note: please add the vendor prefix for older browsers i.e. -moz, -webkit.