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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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Adding Classes

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. So you don't have access to higher-up elements like the <html> tag. If you want to add classes there that can affect the whole document, this is the place to do it.

About the <head>

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. If you need things in the <head> of the document, put that code here.

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Insecure Resource

The resource you are linking to is using the 'http' protocol, which may not work when the browser is using https.

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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Vendor Prefixing

About Vendor Prefixing

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JavaScript

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Babel includes JSX processing.

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HTML 

              
                <div class="content">
  <h1>Vincent van Gogh</h1>
  <p>Vincent van Gogh, in full Vincent Willem van Gogh, (born March 30, 1853, <a href="#" data-hover-content="<div class='hover-content'>
    <img src='https://s3-us-west-2.amazonaws.com/s.cdpn.io/344846/500px-P1040705_copyGemeentehuis_Zundert.jpg' />
    <p>Zundert is a municipality and town in the south of the Netherlands, in the province of North Brabant.</p>
    </div>">Zundert, Netherlands</a> — died July 29, 1890, Auvers-sur-Oise, near <a href="#" data-hover-content="<div class='hover-content'>
    <img src='https://s3-us-west-2.amazonaws.com/s.cdpn.io/344846/eyfel-kulesi-paris.jpg' />
    <p>Paris is the capital and most populous city of France.</p>
    </div>">Paris, France</a>), Dutch painter, generally considered the greatest after <a href="#" data-hover-content="<div class='hover-content'>
    <img src='https://s3-us-west-2.amazonaws.com/s.cdpn.io/344846/Rembrandt.jpg' />
    <p>Rembrandt Harmenszoon van Rijn (July 15, 1606 – October 4, 1669) was a Dutch draughtsman, painter and printmaker.</p>
    </div>">Rembrandt van Rijn</a>, and one of the greatest of the <a href="#" data-hover-content="<div class='hover-content'>
    <img src='https://s3-us-west-2.amazonaws.com/s.cdpn.io/344846/Vincent-van-Gogh-The-Starry-Night-1889-2.jpg' />
    <p>Post-Impressionism is an art movement that developed in the 1890s.</p>
    </div>">Post-Impressionists.</a> The striking colour, emphatic brushwork, and contoured forms of his work powerfully influenced the current of <a href="#" data-hover-content="<div class='hover-content'>
    <img src='https://s3-us-west-2.amazonaws.com/s.cdpn.io/344846/Screen%20Shot%202019-04-20%20at%2012.04.29.png' />
    <p>Expressionism is a modernist movement, initially in poetry and painting, originating in Germany at the beginning of the 20th century.</p>
    </div>">Expressionism</a> in modern art. Van Gogh’s art became astoundingly popular after his death, especially in the late 20th century,
    when his work sold for record-breaking sums at auctions around the world and was featured in blockbuster touring exhibitions. In part because of his extensive published letters, van Gogh has also been mythologized in the popular imagination as the
    quintessential tortured artist.</p>
</div>
<span class="info">Please hover a link</span>
!

CSS 

              
                * {
  box-sizing: border-box;
}

body {
  background-color: #f5f5f5;
  font-family: 'Karla', sans-serif;
}

.content {
  max-width: 550px;
  padding: 40px;
  margin: 40px auto;
}

.content > h1 {
  margin: 0 0 20px 0;
  font-family: 'Cookie', monospace;
  font-size: 60px;
}

.content > p {
  line-height: 1.7;
  margin: 0;
}

.content > p > a {
  color: black;
  font-weight: 700;
}

.content > p > a:hover {
  background-color: #ffe0b2;
}

.hover-content {
  padding: 10px;
  box-shadow: 0 2px 20px rgba(0,0,0,0.2);
  box-sizing: border-box;
  background-color: #fff;
  width: 240px;
}

[data-hover-wrapper] {
  transition: opacity .3s, transform .3s;
}

.hover-content > img {
  max-width: 100%;
  margin-bottom: 10px;
}

.hover-content > p {
  font-size: 14px;
  margin: 0;
  line-height: 1.4;
}

.info {
  position: fixed;
  bottom: 0;
  right: 0;
  background-color: #000;
  padding: 10px 20px;
  color: #fff;
  font-size: 13px;
}
!

JS 

              
                const links = document.getElementsByTagName("a");

[...links].forEach(link => {
  link.addEventListener("mouseover", handleMouseOver);
  link.addEventListener("mousemove", handleMouseMove);
  link.addEventListener("mouseleave", handleMouseLeave);
});

function handlePosition(e) {
  const ID = e.target.getAttribute("data-hover-id");
  const wrapper = document.getElementById(ID);
  let top = "";
  if (
    !(e.target.getBoundingClientRect().top + wrapper.offsetHeight > innerHeight)
  ) {
    top = `${e.clientY + e.target.offsetHeight}px`;
  } else {
    top = `${e.clientY - (wrapper.offsetHeight + e.target.offsetHeight)}px`;
  }

  return `position: fixed; left: ${e.clientX -
    wrapper.offsetWidth / 2}px; top:${top}`;
}

function handleMouseOver(e) {
  const hoverContent = e.target.getAttribute("data-hover-content");
  const ID = Math.random()
    .toString(36)
    .substr(2, 9);
  const wrapper = document.createElement("DIV");
  e.target.setAttribute("data-hover-id", ID);
  wrapper.setAttribute("data-hover-wrapper", "");
  wrapper.setAttribute("id", ID);
  wrapper.setAttribute("style", "opacity: 0; transform: scale(.8)");
  wrapper.innerHTML = hoverContent;
  document.body.append(wrapper);
  wrapper.setAttribute("style", handlePosition(e));
  
  // You can remove this line when you are using. I had added for the demo.
  if (document.querySelector('.info')) document.querySelector('.info').remove();
  
}

function handleMouseLeave(e) {
  const ID = e.target.getAttribute("data-hover-id");
  document.getElementById(ID).style.opacity = 0;
  document.getElementById(ID).style.transform = "scale(.8)";
  setTimeout(() => {
    document.getElementById(ID).remove();
  }, 150);
}

function handleMouseMove(e) {
  const ID = e.target.getAttribute("data-hover-id");
  const wrapper = document.getElementById(ID);
  wrapper.setAttribute("style", handlePosition(e));
}

window.addEventListener('scroll', () => {
  const wrapper = document.querySelector('[data-hover-wrapper]');
  if (wrapper) wrapper.remove();
});
!
999px

Console