How to bevel the corner of a block div?

Until CSS4 border-corner-shape property is in danger! and it's not implemented, This can be done by using CSS3 transforms (to keep border property free for further usage):

HTML:

<div class="box">
  Text Content
</div>

CSS:

.box {
  width: 200px; 
  height: 35px;
  line-height: 35px;
  padding: 0 5px;
  background-color: #ccc;
  padding-right: 20px;
  border: solid 1px black;
  border-right: 0;  
  position: relative;
}

.box:after {
  content: "";
  display: block;
  background-color: #ccc;
  border: solid 1px black;
  border-left: 0;
  width: 35px;
  height: 35px;
  position: absolute;
  z-index: -1;
  top: -1px; /* pull it up because of 1px border */
  right: -17.5px; /* 35px / 2 */
  transform: skew(-45deg);
  -o-transform: skew(-45deg);
  -moz-transform: skew(-45deg);
  -webkit-transform: skew(-45deg);
}

Here is JSBin Demo.

NOTE: There's another div in example above has class attribute of box2, which implemented without using CSS3 declarations that you might consider using it ;)