SOURCE

<div class="cross"></div>

.cross {
    width: 100px;
    height: 100px;
    position: relative;
    margin: 0;
    padding: 0;
    border: 1px solid #ccc;
}

.cross:after{
    content: "";
    display: block;
    position: absolute;
    top: 0px;
    left: 45px;
    width: 10px;
    height: 100%;
    background-color: red;
}

.cross:before {
    content: "";
    display: block;
    position: absolute;
    width: 100%;
    height: 10px;
    background-color: red;
    top: 45px;
    left: 0;
}