<div></div>

body {
    background: #000;
}

div {
    width: 200px;
    height: 200px;

    background:
        linear-gradient(to right, transparent 25px, #0090D9 25px) top / 100% 5px no-repeat,
        linear-gradient(to bottom, transparent 25px, #0090D9 25px) left / 5px 100% no-repeat,
        linear-gradient(135deg, transparent 17px, #0090D9 17px, #0090D9 22px, #fff 20px);

    border: 5px solid #0090D9;
    border-top: none;
    border-left: none;
    border-radius: 6px;
}