<h1>Scroll meh</h1>
<div class="background"><span>Sky</span></div>

@import url('https://fonts.googleapis.com/css?family=Montserrat:900');

html, body {
    margin: 0;
    padding: 0;
    height: 200vh;
    overflow-x: hidden;
    font-family: Montserrat, sans-serif;
}

.background {
    background-image: url(https://images.unsplash.com/photo-1536264911542-668b0180d5a1?ixlib=rb-0.3.5&q=85&fm=jpg&crop=entropy&cs=srgb&ixid=eyJhcHBfaWQiOjE0NTg5fQ&s=a9a32743f06349efc39aeae90f047e9f);
    background-size: cover;
    background-position: 50% 50%;
    background-clip: text;
    height: 200vh;
    font-weight: bold;
    font-size: 50rem;
    -webkit-text-fill-color: transparent;
    position: relative;
    display: flex;
    align-items: flex-start;
    justify-content: flex-end;
    overflow: hidden;
    
    &:before {
        content: '';
        background-image: inherit;
        background-size: cover;
        background-position: 50% 50%;
        position: absolute;
        top: 0;
        right: 0;
        bottom: 0;
        left: 0;
        z-index: -2;
    }
}

h1 {
    position: absolute;
    text-align: center;
    width: 100%;
    color: #fefefe;
}

const background = document.querySelector('.background');

document.addEventListener('scroll', () => {
    const scrollY = window.scrollY;
    
    if(scrollY !== 0) {
        background.style.backgroundPosition = `calc(50% + ${scrollY}px) calc(50% + ${scrollY}px)`;
    } else {
        background.style.backgroundPosition = '';
    }
});