body {
    margin: 0;
    height: 100vh;
    overflow: hidden;
}

/* main page layout */

.main {
    height: inherit;
    background: linear-gradient(dodgerblue, darkblue);
    display: flex;
    align-items: center;
    justify-content: center;
}

.open-popup {
    box-sizing: border-box;
    color: white;
    font-size: 16px;
    font-family: sans-serif;
    width: 10em;
    height: 4em;
    border: 1px solid;
    text-align: center;
    line-height: 4em;
    text-decoration: none;
    text-transform: capitalize;
    margin: 1em;
}

.open-popup:hover {
    border-width: 2px;
}

/* popup page layout */

.popup {
    position: absolute;
    top: 0;
    width: 100%;
    height: inherit;
    flex-direction: column;
    justify-content: flex-start;
    display: none;
}

.popup:target {
    display: flex;
}

.popup .back {
    font-size: 20px;
    font-family: sans-serif;
    text-align: center;
    height: 2em;
    line-height: 2em;
    background-color: #ddd;
    color: black;
    text-decoration: none;
}

.popup .back:visited {
    color: black;
}

.popup .back:hover {
    background-color: #eee;
}

.popup p {
    font-size: 100px;
    text-align: center;
    margin: 0.1em 0.05em;
}

/* animation effects */

.popup > * {
    filter: opacity(0);
    animation: fade-in 0.5s ease-in forwards;
    animation-delay: 1s;
}

@keyframes fade-in {
    to {
        filter: opacity(1);
    }
}

.popup::before {
    content: '';
    position: absolute;
    width: 100%;
    height: 0;
    top: 50%;
    background-color: white;
    animation: open-animate 0.5s cubic-bezier(0.8, 0.2, 0, 1.2) forwards;
    animation-delay: 0.5s;
}

@keyframes open-animate {
    to {
        height: 100vh;
        top: 0;
    }
}

.popup::after {
    content: '';
    position: absolute;
    width: 0;
    height: 2px;
    background-color: white;
    top: calc((100% - 2px) / 2);
    left: 0;
    animation: line-animate 0.5s cubic-bezier(0.8, 0.2, 0, 1.2);
}

@keyframes line-animate {
    50%, 100% {
        width: 100%;
    }
}