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HTML

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HTML preprocessors can make writing HTML more powerful or convenient. For instance, Markdown is designed to be easier to write and read for text documents and you could write a loop in Pug.

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About the <head>

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Insecure Resource

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CSS

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CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

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CSS Base

About CSS Base

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JavaScript

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JavaScript preprocessors can help make authoring JavaScript easier and more convenient.

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Babel includes JSX processing.

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HTML 

              
                <!--
Demo from an article on my personal website about the CSS Variables: https://xtrp.io/blog/2020/02/05/scroll-progress-bar-in-js-and-css/.

Give this CodePen a ❤️ if you liked it and check out the article :)
-->

<div class="post-container">
  <!-- progress bar div -->
  <div class="progress-bar-container">
    <div class="progress-bar-container__progress"></div>
  </div>
  
  <!-- post content below progress bar -->
  <h1>Post Title</h1>
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>
  <h2>Heading #1</h2>
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>
  <h2>Heading #2</h2>
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>
  <h2>Heading #3</h2>
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>
  <h2>Heading #4</h2>
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>
  <h2>Heading #5</h2>
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>
  <h2>Heading #6</h2>
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>
  <h2>Heading #7</h2>
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>
  <h2>Heading #8</h2>
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>
  <h2>Heading #10</h2>
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>
</div>
!

CSS 

              
                /* default CSS variables */
:root {
  --text-color: #222222;
  --progress-color: #2ecc71;
  --progress-height: .5rem;
}

/* default styles */
* {
  margin: 0;
  padding: 0;
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, 'Open Sans', 'Helvetica Neue', sans-serif;
}
body {
  font-size: 18px;
}

/* progress bar container */
.progress-bar-container {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: var(--progress-height);
}

/* progress bar */
.progress-bar-container__progress {
  height: var(--progress-height);
  background-color: var(--progress-color);
  width: 0%;
  float: left;
}

/* post content and container */
.post-container {
  padding: 2rem 3rem;
  box-sizing: border-box;
  height: 100vh;
  overflow: scroll;
}
.post-container h1,
.post-container h2 {
  line-height: 1.35;
  margin-bottom: 1.5rem;
}
.post-container p {
  line-height: 2;
  margin-bottom: 1.5rem;
}
!

JS 

              
                // variables for progress bar and post container elements
const progressContainerEl = document.querySelector(".post-container");
const progressBarEl = document.querySelector(".progress-bar-container__progress");

// function to check scroll position and update scroll progress bar accordingly
const updateScrollProgressBar = () => {
  // get full scroll height
  const scrollHeight = progressContainerEl.scrollHeight - heightInViewport(progressContainerEl);
  console.log(scrollHeight);
  // get current scroll position
  const scrollPosition = progressContainerEl.scrollTop;
  
  // get scroll percentage and set width of progress bar
  const scrollPercentage = (scrollPosition / scrollHeight) * 100;
  progressBarEl.style.width = scrollPercentage + "%";
}

// bind window onload and onscroll events to update scroll progress bar width
progressContainerEl.addEventListener("scroll", updateScrollProgressBar)
progressContainerEl.addEventListener("load", updateScrollProgressBar)

// function to get visible height in viewport
// some code taken from user Roko C. Buljan on https://stackoverflow.com/questions/24768795/get-the-visible-height-of-a-div-with-jquery
function heightInViewport(el) {
    var elH = el.offsetHeight,
        H   = document.body.offsetHeight,
        r   = el.getBoundingClientRect(), t=r.top, b=r.bottom;
    return Math.max(0, t>0? Math.min(elH, H-t) : Math.min(b, H));
}
!
999px

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