Simple scroll up script for html websites.

scrollUp.html

<button type="button" id="topButton" onclick="topFunction()">Top</button>
<style>
    #topButton {
        display: none;
        position: fixed;
        bottom: 25px;
        right: 30px;
        z-index: 99;
    }
</style>
<script>
    var mybutton = document.getElementById('topButton');
    window.onscroll = function() {
        scrollFunction()
    };
    function scrollFunction() {
        if (document.body.scrollTop > 20 || document.documentElement.scrollTop > 20) {
            mybutton.style.display = 'block';
        } else {
            mybutton.style.display = 'none';
        };
    };
    function topFunction() {
        document.body.scrollTop = 0;
        document.documentElement.scrollTop = 0;
    };
</script>