How to achieve chamfered CSS Border Corners rather than rounded corners?

This is what you need, transparency and everything

.left,
.right {
  width: 100px;
  height: 100px;
  float: left;
  position: relative;
  overflow: hidden;
}

.right::after,
.left::after {
  content: '';
  width: 200px;
  height: 200px;
  position: absolute;
  -moz-transform: rotate(45deg);
  -webkit-transform: rotate(45deg);
  -o-transform: rotate(45deg);
  -ms-transform: rotate(45deg);
  transform: rotate(45deg);
}

.right::after {
  background: lightblue;
  left: -40px;
  top: -100px;
}

.left::after {
  background: lightpink;
  left: -60px;
  top: -100px;
}
<div class="left"></div>
<div class="right"></div>

CSS3 Gradients can do the trick:

Try this, Here's a Fiddle.

div {
  background: #c00;
  /* fallback */
  background: -moz-linear-gradient(45deg, transparent 10px, #c00 10px), -moz-linear-gradient(135deg, transparent 10px, #c00 10px), -moz-linear-gradient(225deg, transparent 10px, #c00 10px), -moz-linear-gradient(315deg, transparent 10px, #c00 10px);
  background: -o-linear-gradient(45deg, transparent 10px, #c00 10px), -o-linear-gradient(135deg, transparent 10px, #c00 10px), -o-linear-gradient(225deg, transparent 10px, #c00 10px), -o-linear-gradient(315deg, transparent 10px, #c00 10px);
  background: -webkit-linear-gradient(45deg, transparent 10px, #c00 10px), -webkit-linear-gradient(135deg, transparent 10px, #c00 10px), -webkit-linear-gradient(225deg, transparent 10px, #c00 10px), -webkit-linear-gradient(315deg, transparent 10px, #c00 10px);
}

div {
  background-position: bottom left, bottom right, top right, top left;
  -moz-background-size: 50% 50%;
  -webkit-background-size: 50% 50%;
  background-size: 50% 50%;
  background-repeat: no-repeat;
}


/* Ignore the CSS from this point, it's just to make the demo more presentable */

div {
  float: left;
  width: 50px;
  margin: 15px auto;
  padding: 15px;
  color: white;
  line-height: 1.5;
}
<div>Div 1</div>
<div>Div 2</div>

This is also possible using "clip-path".

-webkit-clip-path: polygon(20% 0%, 80% 0%, 100% 20%, 100% 80%, 80% 100%, 20% 100%, 0% 80%, 0% 20%);
clip-path: polygon(20% 0%, 80% 0%, 100% 20%, 100% 80%, 80% 100%, 20% 100%, 0% 80%, 0% 20%);

div {
  width: 200px;
  height: 200px;
  background: red;
  -webkit-clip-path: polygon(20% 0%, 80% 0%, 100% 20%, 100% 80%, 80% 100%, 20% 100%, 0% 80%, 0% 20%);
  clip-path: polygon(20% 0%, 80% 0%, 100% 20%, 100% 80%, 80% 100%, 20% 100%, 0% 80%, 0% 20%);
}
<div></div>

Source Codepen

Support for clip-path can be found here... http://caniuse.com/#search=clip-path


Here's a way, although it does have some shortcomings, like no borders and it isn't transparent:

.left,
.right {
  width: 100px;
  height: 100px;
  float: left;
  position: relative;
}

.left {
  background: lightpink;
}

.right {
  background: lightblue;
}

.right::after,
.left::after {
  width: 0px;
  height: 0px;
  background: #fff;
  content: '';
  position: absolute;
  bottom: 0;
}

.right::after {
  left: 0;
  border-top: 10px solid lightblue;
  border-right: 10px solid lightblue;
  border-left: 10px solid white;
  border-bottom: 10px solid white;
}

.left::after {
  right: 0;
  border-top: 10px solid lightpink;
  border-right: 10px solid white;
  border-left: 10px solid lightpink;
  border-bottom: 10px solid white;
}
<div class="left"></div>
<div class="right"></div>

RESULT:

enter image description here

Here's a fiddle.