How to create triangle shape in the top-right angle of another div to look divided by border

Question

I want to create shape like on this picture:

I created triangle shape like on this pic, and s

Answer 1

You could use a rotate pseudo element in conjunction to an overflow:hidden on the parent.

From here you could position the pseudo to the top right using position:absolute and you should be good to go!

div {
  height: 250px;
  width: 300px;
  background: lightgray;
  border-radius: 10px;
  border: 5px solid dimgray;
  position: relative;
  overflow: hidden;
  margin: 30px auto;
}
div:before {
  content: "";
  position: absolute;
  top: -60px;
  right: -60px;
  height: 100px;
  width: 100px;
  background: green;
  border: 5px solid dimgray;
  transform: rotate(45deg);
}

/***********FOR DEMO ONLY*******************/


html, body {
    margin:0;
    padding:0;height:100%;
    vertical-align:top;overflow:hidden;
    background: rgb(79, 79, 79);
    background: -moz-radial-gradient(center, ellipse cover, rgba(79, 79, 79, 1) 0%, rgba(34, 34, 34, 1) 100%);
    background: -webkit-gradient(radial, center center, 0px, center center, 100%, color-stop(0%, rgba(79, 79, 79, 1)), color-stop(100%, rgba(34, 34, 34, 1)));
    background: -webkit-radial-gradient(center, ellipse cover, rgba(79, 79, 79, 1) 0%, rgba(34, 34, 34, 1) 100%);
    background: -o-radial-gradient(center, ellipse cover, rgba(79, 79, 79, 1) 0%, rgba(34, 34, 34, 1) 100%);
    background: -ms-radial-gradient(center, ellipse cover, rgba(79, 79, 79, 1) 0%, rgba(34, 34, 34, 1) 100%);
    background: radial-gradient(ellipse at center, rgba(79, 79, 79, 1) 0%, rgba(34, 34, 34, 1) 100%);
    filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#4f4f4f', endColorstr='#222222', GradientType=1);
}