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About the <head>

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CSS

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CSS Base

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JavaScript

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Babel includes JSX processing.

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HTML 

              
                
<body>
    <div class="progress-bar" id="progressBar"></div>
    <section class="main">
        <p class="title">
            Title 1
        </p>
        <p class="text">
            Lorem ipsum dolor sit amet consectetur adipisicing elit. Odio quasi, maiores ullam assumenda a veniam quia
            exercitationem ratione dolorum nihil cupiditate totam consequatur accusantium quos temporibus doloribus
            nobis perspiciatis repudiandae!Expedita minus repudiandae odio voluptate placeat vitae ut quasi incidunt
            doloribus porro officiis dicta sunt, corrupti libero nulla sint saepe consequuntur eum quae aliquam
            accusamus mollitia voluptates? Dolorum, blanditiis autem?
        </p>
        <p class="title">
            Title 2
        </p>
        <p class="text">
            Lorem ipsum dolor sit amet consectetur adipisicing elit. Consectetur dolor ipsa velit libero laboriosam cum
            accusamus sint dolore repellat. Accusamus tempore odio totam vitae quasi excepturi sed necessitatibus?
            Pariatur, cumque!
            Distinctio molestiae aliquam in repellendus laborum, facere quo reiciendis obcaecati cum? Minima fuga quis,
            commodi quod aperiam enim ad praesentium reiciendis laborum tempore, corrupti, quam sunt suscipit
            consequuntur architecto aspernatur!
            Sed nostrum earum accusantium iste quibusdam, delectus odit fuga, ex, cumque debitis consectetur molestias
            deserunt quo. Deserunt necessitatibus perferendis vel explicabo quibusdam earum consequatur facilis minus
            culpa. Eos, quod. Illo!
            Aperiam fuga provident voluptas quo. Iure, laboriosam quibusdam alias sint est nam adipisci architecto,
            inventore deserunt tenetur, omnis minus aliquam reiciendis numquam! Maxime necessitatibus repellat officiis
            laudantium sit ut impedit.
        </p>
        <p class="title">
            Title 3
        </p>
        <p class="text">
            Lorem ipsum dolor sit, amet consectetur adipisicing elit. Obcaecati tempore repellat aspernatur explicabo,
            facilis, voluptatum nulla perferendis illo non, distinctio officiis tempora id neque ut natus saepe a eaque
            velit?
            Eveniet praesentium qui dolores, corrupti temporibus sunt odit sequi velit debitis, tempora et non
            assumenda? Inventore quisquam non iusto praesentium delectus, aspernatur voluptas ipsa doloribus, explicabo
            voluptatum repellat dolorum reprehenderit!
            Magnam dolor praesentium eveniet tempore qui ratione ipsam esse! Neque quis, at illum assumenda doloremque
            dignissimos ipsum. Commodi, iure. Odit ipsam quod similique labore nemo animi nihil totam beatae dolorem!
            Aspernatur, magni nesciunt accusamus repudiandae, ad quis consequatur error qui pariatur, natus repellendus
            ratione. Est amet hic fugiat! Veritatis, quia blanditiis aut aperiam quod aliquam qui iusto nesciunt
            doloremque tenetur.
            Iusto, atque veniam iste numquam reprehenderit officia saepe. Sed, tempore ex ducimus obcaecati eum, qui
            neque nulla exercitationem, doloribus perferendis quae incidunt aperiam veritatis vel aspernatur. Sed illum
            minus dolores?
            Neque expedita odit delectus ea placeat. Neque ratione repellat tempora hic. Tempore culpa ipsam ex iusto
            dolore mollitia rerum quia cumque adipisci. Odio sed corporis rerum, repellat tempore sequi accusantium?
            Inventore eius et earum laboriosam minima reprehenderit neque. Repudiandae voluptate deleniti odio
            exercitationem laudantium officiis sapiente? Quidem, aliquid porro placeat ipsum suscipit nihil veritatis
            saepe magni eveniet quis praesentium ex?
            Doloribus rem asperiores accusantium quidem impedit ab, saepe modi repellat nesciunt earum reprehenderit hic
            dolorem, dolores itaque? Necessitatibus laboriosam sequi esse fugit, quos earum, aliquam architecto aperiam,
            nam accusamus delectus.
        </p>
        <p class="title">
            Title 4
        </p>
        <p class="text">
            Lorem ipsum dolor sit amet consectetur adipisicing elit. Necessitatibus at error recusandae tenetur pariatur
            libero odit, totam ea. Maxime corporis atque maiores suscipit doloribus autem esse exercitationem eaque
            quibusdam necessitatibus.
            Illum, facere! Autem nostrum, officia dicta ipsa repellendus distinctio eum rerum? Illo iure totam sit quae
            reiciendis ipsa, voluptatum culpa, harum nostrum est delectus deserunt tenetur qui vitae maiores possimus.
            Repudiandae consequatur quis cum doloremque error dolorum omnis, eaque distinctio qui fugit impedit sunt et
            nemo, at voluptatum recusandae corporis sapiente aspernatur, repellat dignissimos voluptatibus earum quae
            facere tempore? Mollitia?
            Officiis, omnis doloremque id numquam cumque accusamus aperiam culpa maiores sit adipisci beatae
            necessitatibus ipsa illum voluptatem vel facere dignissimos repudiandae placeat porro natus perspiciatis
            voluptatum rerum! Cum, voluptas ut.
            Voluptate, cum, inventore numquam autem tenetur quidem omnis labore consectetur, rem reiciendis delectus
            pariatur culpa cupiditate eligendi architecto incidunt corrupti dignissimos! A, nobis quo? Iusto provident
            in alias maiores dolores!
            Corporis, ad eveniet repellendus consequatur assumenda rem inventore? Officia quae, reprehenderit explicabo
            modi nihil eligendi, itaque, consequuntur dolor perspiciatis ducimus quas hic. Aperiam cumque voluptas
            voluptates, incidunt ad libero ipsam.
            Labore repellat nulla praesentium libero tempore hic voluptas ipsum. Porro reprehenderit eius, iste
            quisquam, praesentium consectetur nihil rem eos explicabo cum quia natus sapiente vero aliquam? Cum incidunt
            odit fuga!
            Esse, molestiae nesciunt temporibus optio modi atque blanditiis sint id, deserunt cum nemo est quibusdam
            repellendus laudantium rerum autem ipsam! Numquam ipsam, expedita ipsa illum deleniti et delectus dolore
            maiores?
            Alias similique, vitae ullam dolorem doloribus aut cum magni odio rem aliquam. Laboriosam saepe eaque
            reiciendis voluptatibus? Omnis odio, mollitia voluptates ipsam minus similique perspiciatis architecto nulla
            eum dolore saepe!
            Magnam sint facere nam, qui nisi fugit. Omnis explicabo accusantium nemo impedit. Vel repudiandae et illum
            eius minus sapiente maxime deserunt laudantium. Nobis laudantium quis odit fugiat totam porro nesciunt?
            Iusto ipsum incidunt expedita adipisci! Velit, culpa. At laborum deserunt illum reprehenderit eligendi
            beatae eius corporis, qui autem iste quis tenetur laudantium ratione labore doloribus consequuntur atque
            fugit aspernatur provident.
            Placeat tempora impedit laudantium amet ut, facere fugiat, vel ducimus nesciunt pariatur dignissimos non!
            Enim, non ullam magnam omnis veritatis, sunt quos hic incidunt odio aut perspiciatis amet id vero.
        </p>
    </section>


   
</body>
!

CSS 

              
                body {
    margin: 0;
    padding: 20px;
    font-family: 'Roboto';
}

.title {
    font-size: 36px;
}

.text {
    font-size: 22px;
    margin-bottom: 100px;
}


.progress-bar {
    position: fixed;
    left: 0;
    top: 0;
    height: 4px;
    width: 0%;
    background: red;
}
!

JS 

              
                function progressBar() {
    let scroll = document.body.scrollTop || document.documentElement.scrollTop;
    let height = document.documentElement.scrollHeight - document.documentElement.clientHeight;
    let scrolled = scroll / height * 100;

    document.getElementById('progressBar').style.width = scrolled + '%';
}

window.addEventListener('scroll', progressBar);
!
999px

Console