style.css

@import url(https://fonts.googleapis.com/css?family=Poppins:100,100italic,200,200italic,300,300italic,regular,italic,500,500italic,600,600italic,700,700italic,800,800italic,900,900italic);
* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: 'Poppins', sans-serif;
}
body {
    display: flex;
    align-items: center;
    justify-content: center;
    min-height: 100vh;
    background-color: #f1f1f1;
}

.content {
    padding: 0 5rem;
}

.popup {
    position: fixed;
    top: 0;
    width: 100vw;
    height: 100vh;
    background-color: rgba(0, 0, 0, .3);
    display: grid;
    place-content: center;
    opacity: 0;
    pointer-events: none;
    transition: 200ms ease-in-out opacity;
}
.popup-content {
    width: clamp(300px, 90vw, 500px);
    background-color: #fff;
    padding: clamp(1.5rem, 100vw, 3rem);
    box-shadow: 0 0 .5em rgba(0, 0, 0, .5);
    border-radius: .5em;
    opacity: 0;
    transform: translateY(20%);
    transition: 200ms ease-in-out opacity,
                200ms ease-in-out transform;
    position: relative;
}
.popup h1 {
    position: absolute;
    top: 2rem;
    right: 2rem;
    line-height: 1;
    cursor: pointer;
    user-select: none;
}
.popup h1:active {
    transform: scale(.9);
}

.showPopup {
    opacity: 1;
    transform: translateY(0);
    pointer-events: all;
}