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HTML

              
                progress.c-progress.js-progress(value="0" max="1")

article
    h1 Mobile-friendly reading progress with minimal JS
    each section in [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 ]
        p Lorem ipsum dolor sit amet, consectetur adipiscing elit. Proin porta risus urna, non molestie risus dignissim ut. Maecenas varius leo quis lectus rhoncus mollis. Donec condimentum, ipsum ac consectetur rhoncus, tellus dolor volutpat libero, vitae volutpat mi eros a metus. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Proin at dolor non enim pretium tempor id gravida dui. Pellentesque porta metus in felis molestie gravida. Sed in diam mi. Suspendisse vel odio dapibus, aliquam nisi ut, condimentum ante.

        p In tincidunt dui ac ante efficitur posuere. Integer diam nisl, scelerisque sit amet est non, venenatis suscipit tortor. Integer ultricies bibendum ante et tempus. Donec nibh sem, tempor ac consequat at, rhoncus at elit. Fusce aliquam tempus ex, vitae venenatis ipsum mattis aliquet. Aliquam convallis lacinia lectus, sit amet consectetur augue aliquam eu. Morbi enim mi, scelerisque non viverra in, mattis vitae ligula. Donec nulla lacus, interdum ac tortor vitae, consectetur sodales lorem. Morbi iaculis ante sit amet lacus maximus, non elementum libero scelerisque. Vivamus id magna convallis, ullamcorper nisl eu, efficitur ipsum. Praesent eros ante, maximus et quam vitae, laoreet blandit neque.
              
            
!

CSS

              
                $progress-color: lighten(#1e1935, 20)

.c-progress
    position: fixed
    top: 0
    left: 0
    width: 100%
    height: 5px
    appearance: none
    background: transparent
    border: none
    color: $progress-color
    z-index: 5

    &::-webkit-progress-bar
        background: transparent

    &::-webkit-progress-value
        background: $progress-color

    &::-moz-progress-bar
        background: $progress-color

    
article
    margin: 0 auto
    width: 65%
    max-width: 700px
    
p
    font: 16px/#{1.5} "Poppins", Arial, sans-serif
    margin-bottom: 2em

h1
    font: bold 46px/#{1.25} "Poppins", Arial, sans-serif
    margin: 1.5em 0

*
    -webkit-font-smoothing: antialisased
    box-sizing: border-box
              
            
!

JS

              
                //- Reading Progress Indicator

{
    const progress = document.querySelector('.js-progress');
    const body = document.body;
    const html = document.documentElement;

    const updateSizes = () => ({
        height: Math.max(body.scrollHeight, body.offsetHeight, html.clientHeight, html.scrollHeight, html.offsetHeight),
        vh: Math.max(html.clientHeight, window.innerHeight || 0)
    });

    let scrollY = 0;
    let sizes = updateSizes();

    const update = () => progress.setAttribute('value', 100 - (sizes.height - scrollY - sizes.vh) / sizes.height * 100);

    const onScroll = () => {
        scrollY = (window.pageYOffset || document.scrollTop) - (document.clientTop || 0);
        requestAnimationFrame(update);
    }

    const onResize = () => {
        sizes = updateSizes();
        requestAnimationFrame(update);
    }

    if(progress) {
        progress.setAttribute('max', 100);
        window.addEventListener('scroll', onScroll, false);
        window.addEventListener('resize', onResize, false);
    }
}
              
            
!
999px

Console