I’d suggest, given the following mark-up:
#box {
width: 10em;
height: 6em;
border: 4px solid #ccc;
background-color: #fff;
position: relative;
}
#box::before,
#box::after {
content: '';
position: absolute;
top: 0;
right: 0;
border-color: transparent;
border-style: solid;
}
#box::before {
border-width: 1.5em;
border-right-color: #ccc;
border-top-color: #ccc;
}
#box::after {
border-radius: 0.4em;
border-width: 1.35em;
border-right-color: #0c0;
border-top-color: #0c0;
}
<div id="box"></div>