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HTML

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About the <head>

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Insecure Resource

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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 

              
                <header>
  <h1>Lepljivi naslov</h1>
</header>

<section>
  <div>Lorem ipsum dolor sit amet consectetur adipisicing elit. Dicta ipsum ipsam sapiente dolores quibusdam assumenda illum totam vitae consequatur, natus omnis culpa blanditiis reiciendis fugit laboriosam. Nostrum repudiandae ducimus deserunt!</div>
  <div>Cupiditate voluptate impedit sequi porro, commodi aperiam ratione vitae similique voluptas dicta perspiciatis quisquam hic ducimus nulla quam exercitationem odit rerum. Aut autem facere corporis tenetur possimus maiores adipisci nulla!</div>
  <div>Voluptatum maxime labore nulla, corporis maiores, facere qui numquam sapiente ipsum voluptatibus magnam! A, vel id nobis cupiditate ut corrupti, odio similique nulla maxime amet autem enim? Labore, vel sit!</div>
  <div>Laborum illo ab repudiandae quis voluptates praesentium magni non obcaecati quibusdam, sequi, a doloremque nemo eligendi sapiente. Omnis, ad? Eveniet debitis recusandae sequi est id magni, cum doloribus minima ratione.</div>
  <div>Officiis nisi, ab vel cumque amet voluptatem rem ducimus quaerat in unde, et, obcaecati officia vero at nam expedita ea aspernatur id cupiditate dolore corporis? Doloribus ipsum exercitationem quae rerum?</div>
 <div>Lorem ipsum dolor sit amet consectetur adipisicing elit. Possimus ea, necessitatibus cupiditate exercitationem distinctio sint expedita quos quibusdam soluta nisi sed? Ut aliquid eaque expedita alias quos laboriosam natus illum!</div>
 <div>Tenetur aspernatur totam modi, quaerat consequatur, aperiam veritatis at esse officiis, enim consequuntur hic earum incidunt voluptate quis fugit facere ex corrupti! Tempore sequi harum a blanditiis obcaecati. Quod, nemo!</div>
 <div>Sunt reiciendis neque distinctio animi, maxime sed minus, atque natus eum saepe, fuga ut et at repellendus cupiditate deserunt perferendis iure rem voluptate commodi ipsum mollitia ullam debitis. Iusto, ex?</div>
 <div>Consequuntur dicta architecto doloremque harum nostrum exercitationem repellendus quasi dolorem quis quidem, adipisci neque quam inventore nihil nobis labore in dolore ad excepturi quia officiis impedit accusantium eos. Sint, ullam.</div>
 <div>Unde beatae, iste ipsam mollitia quibusdam, magnam aut tenetur quam possimus laborum rem, adipisci culpa quia saepe facilis laboriosam natus. Saepe voluptate placeat ratione at, velit eligendi aspernatur ipsum nam.</div>
</section>
!

CSS 

              
                header {
  height: 200px;
  text-align: center;
  background: black;
}

h1 {
  margin: 0;
  position: relative;
  top: 50%;
  transform: translateY(-50%);
  z-index: 1;
  color: white;

  &.fix {
    margin: 0;
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    line-height: 1.5;
    transform: translateY(0);
    background: black;
  }
}
!

JS 

              
                var header = document.querySelector('header');
var naslov = header.querySelector('h1');
var visinaHeadera = getComputedStyle(header).height.split('px')[0];
var visinaNaslova = getComputedStyle(naslov).height.split('px')[0];
var rastojanjeOdNaslovaDoVrhaHeadera= ((visinaHeadera - visinaNaslova) / 2);

window.addEventListener('scroll', function (e) {
  var velicinaScrolla = window.scrollY;
  if( velicinaScrolla > rastojanjeOdNaslovaDoVrhaHeadera) {
    naslov.classList.add("fix");
  }
  if( velicinaScrolla < rastojanjeOdNaslovaDoVrhaHeadera) {
    naslov.classList.remove("fix");
  }
}, false);
!
999px

Console