* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    background: crimson;
}

.main {
    width: 100%;
    height: 100vh;
    position: relative;
}

a {
    width: 90px;
    height: 35px;
    border: 1px solid black;
    color: black;
    text-decoration: none;
    position: absolute;
    padding: 8px;
    right: 0;
    bottom: 0;
    left: 0;
    top: 0;
    margin: auto;
}

a:hover {
    background: darkgray;
}