JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css">

<span id="toTop"><i class="fa fa-angle-up" aria-hidden="true"></i></span>
<div class="con">
    <section id="b2">
        <!-- Entrées -->
        <h3 class="fade">ENTRÉES</h3><br>
        <article class="plats">
            <div class="left">
                <p class="fade">MILLEFEUILLE BUFFALA</p><br>
                <p class="fade">ASSIETTE DE LÉGUMES GRILLÉS<br>
                    <em>fleur de sel de Guérande et huile d’olive d’Italie</em></p><br>
                <p class="fade">SALADE DE CALAMARS<br>
                    <em>calamar, rucola, tomates</em></p><br>
                <p class="fade">SALADE DE POULPE</p><br>
            </div>
            <div class="right">
                <p class="fade">ANTIPASTO CASA NOSTRA<br>
                    <em>foccaccia, burrata, petits légumes grillés, Vitello Tonnato, etc</em></p><br>
                <p class="fade">CARPACCIO DE BOEUF<br>
                    <em>rucola, grana padano, champignons</em></p><br>
                <p class="fade">BRESAOLA, RUCOLA ET GRANA PADANO</p><br>
                <p class="fade">RUCOLA PARMESAN</p><br>
            </div>
        </article>
    </section>

    <section id="b3">
        <!-- Pâtes -->
        <h3 class="fade">PASTA</h3><br>
        <article class="plats">
            <div class="left">
                <h4>LINGUINE</h4><br>
                <p class="fade">ALLO SCOGLIO<br>
                    <em>fruits de mer, tomates</em></p><br>
                <p class="fade">ALLE VONGOLE VERACE<br>
                    <em>vongoles, persil, ail, huile d’olive</em></p><br>
                <h4>PENNE</h4><br>
                <p class="fade">ALLA SICILLIANA<br>
                    <em>tomates, aubergines, olives noires, mozzarella</em></p><br>
                <p class="fade">ALL’ARRABIATA<br>
                    <em>tomates, piment, ail</em></p><br>
            </div>
        ...

SCSS

span#toTop {
    position: fixed;
    z-index: 10;
    left: 2.1em;
    bottom: 5em;
    opacity: 0;
    transition: all .3s ease-in-out;
    &.show {
        opacity: 1;
    }
    .fa.fa-angle-up {
        font-size: 6em;
        color: #f00;
        transition: .1s all 0s;
        &:hover {
            cursor: pointer;
        }
        &:active {
            transform: scale(1.2)
        }
    }
}

section {
    text-align: center;
    background-color: lightgrey;
    margin: .25em auto;
    padding: .25em;
    width: 1000px;
}

JavaScript

(function() { // reveal arrow
    var arrow = document.getElementById('toTop'),
        myPage = document.querySelectorAll('html, body'),
        revealArrrow = function() {
            var y = window.scrollY;
            y >= 600 ? arrow.className = 'show' : element.classList.remove('show');
        };
    arrow.addEventListener('click', function(e) {
        myPage.animate({
            scrollTop: 0
        }, 2000, 'easeInOutQuart');
    });
    window.addEventListener('scroll', revealArrrow);
})();
EasingFunctions = {
    easeInOutQuart: function(t) {
        return t < 0.5 ? 8 * t * t * t * t : 1 - 8 * (--t) * t * t * t
    }
}