style.css

html{
    box-sizing: border-box;
    background: #ffc600;
    font-family: "Helvetica Neue",Helvetica,Arial;
    font-size:20px;
    font-weight:200;
}

body{
    margin: 0;
}

*, *:before, *:after{
    box-sizing: inherit;
}

.panels{
    min-height: 100vh;
    overflow: hidden;
    display: flex;
}

.panel{
    background: #6b0f9c;
    box-shadow: inset 0 0 0 5px rgba(255, 255, 255, 0.1);
    color: white;
    text-align: center;
    /* animation */
    transition: font-size 0.7s cubic-bezier(0.61,-0.19, 0.7,-0.11),
    flex 0.7s cubic-bezier(0.61,-0.19, 0.7,-0.11),
    background 0.2s;

    font-size:20px;
    background-size: cover;
    background-position: center;
    flex: 1;
    justify-content: center;
    display: flex;
    flex-direction: column;
}


.panel1 { background-image:url(https://source.unsplash.com/gYl-UtwNg_I/1500x1500); }
.panel2 { background-image:url(https://source.unsplash.com/rFKUFzjPYiQ/1500x1500); }
.panel3 { background-image:url(https://images.unsplash.com/photo-1465188162913-8fb5709d6d57?ixlib=rb-0.3.5&q=80&fm=jpg&crop=faces&cs=tinysrgb&w=1500&h=1500&fit=crop&s=967e8a713a4e395260793fc8c802901d); }
.panel4 { background-image:url(https://source.unsplash.com/ITjiVXcwVng/1500x1500); }
.panel5 { background-image:url(https://source.unsplash.com/3MNzGlQM7qs/1500x1500); }

/* Flex items */
.panel > *{
    margin: 0;
    width: 100%;
    transition:transform 0.5s;
    flex: 1 0 auto; 
    display: flex;
    justify-content: center;
    align-items: center;
}

.panel > *:first-child {transform: translateY(-100%)}
.panel.open-active > *:first-child {transform: translateY(0); }
.panel > *:last-child {transform: translateY(100%)}
.panel.open-active > *:last-child {transform: translateY(0); }

.panel p{
    text-transform: uppercase;
    font-family: 'Amatic SC', cursive;
    text-shadow: 0 0 4px rgba(0, 0, 0, 0.72), 0 0 14px rgba(0, 0, 0, 0.45);
    font-size: 2em;
}

.panel p:nth-child(2){
    font-size: 4em;
}

.panel.open{
    flex: 5;
    font-size:40px;
}

@media only screen and (max-width: 600px) {
    .panel p{
        font-size: 1em;
    }
}