body {
    height:100%;
    margin:0;
    padding:0;
}
body * {
    box-sizing: border-box;
}

.cards {
    display: flex;
    flex-wrap: wrap;
    height: 100vh;
    margin: 0;
    padding-left: 20px;
    padding-top: 20px;
    list-style: none;
}
.cards li {
    width: 25%;
    height: 50%;
    padding-bottom: 20px;
    padding-right: 20px;
}
.image {
    background-size: cover;
    background-position: center;
    width: 100%;
    height: 100%;
    cursor: pointer;
}
.image:hover {
    opacity: 0.7;
}

/* Media Queries */
@media (max-width: 900px) {
    main .card { width: 50%; }
}

@media (max-width: 550px) {
    main .card { width: 100%; }
}