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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

About CSS Preprocessors

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

It's a common practice to apply CSS to a page that styles elements such that they are consistent across all browsers. We offer two of the most popular choices: normalize.css and a reset. Or, choose Neither and nothing will be applied.

Vendor Prefixing

About Vendor Prefixing

To get the best cross-browser support, it is a common practice to apply vendor prefixes to CSS properties and values that require them to work. For instance -webkit- or -moz-.

We offer two popular choices: Autoprefixer (which processes your CSS server-side) and -prefix-free (which applies prefixes via a script, client-side).

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JavaScript

About JavaScript Preprocessors

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

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Babel includes JSX processing.

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Packages

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HTML 

              
                <h2>Bulletproof Pure CSS Tooltips</h2>

<main>
  <p>Use the range slider below to adjust the body font size. The tooltip uses em units (which are based on the pixels of the parent font size), so the size, padding, and position of the tooltip stays relative to the link.<p>
  
  <p class="tt">Lorem ipsum dolor sit amet <a href="#" data-tooltip="This is a pure CSS tooltip">hover over this link</a> elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua ut enim ad minim veniam, <a href="#" data-tooltip="This is a pure CSS tooltip">hover over this link which is longer than the other</a> quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p>
  
  <p>You'll also notice at certain sizes the link spans two lines and the tooltip still appears at a comfortable location.</p>

  <p class="range"><input type="range" min="10" max="30" step="1" value="20" id="slider"></p>
</main>
!

CSS 

              
                body {
  font-size: 24px;
  line-height: 1.4;
  padding: 0 20px;
}

*, *::before, *::after {
  box-sizing: border-box;
}

h2 {
  text-align: center;
  font-size: 30px;
}

main {
  margin: 0 auto;
  max-width: 650px;
}

.tt {
  color: firebrick;
}

.range {
  text-align: center;
  font-size: 20px;
  position: absolute;
  bottom: 0;
  left: 30px;
  right: 30px;
}

a[data-tooltip]:link, a[data-tooltip]:visited {
  position: relative;
  text-decoration: none;
	border-bottom: solid 1px;
}

a[data-tooltip]:before {
	content: "";
	position: absolute;
	border-top: 1em solid #0090ff;
	border-left: 1.5em solid transparent;
	border-right: 1.5em solid transparent;
	display: none;
  top: -1em;
}

a[data-tooltip]:after {
	content: attr(data-tooltip);
	position: absolute;
	color: white;
	top: -2.5em;
  left: -1em;
	background: #0090ff;
	padding: .25em .75em;
	border-radius: .5em;
	white-space: nowrap;
	display: none;
}

a[data-tooltip]:hover:before, a[data-tooltip]:hover:after {
	display: inline;
}

/* credit: https://css-tricks.com/styling-cross-browser-compatible-range-inputs-css/ */
input[type=range] {
  -webkit-appearance: none;
  width: 100%;
}

input[type=range]:focus {
  outline: none;
}

input[type=range]::-webkit-slider-runnable-track {
  width: 100%;
  height: 8.4px;
  cursor: pointer;
  animate: 0.2s;
  box-shadow: 1px 1px 1px #000000, 0px 0px 1px #0d0d0d;
  background: #0090ff;
  border-radius: 1.3px;
  border: 0.2px solid #010101;
}

input[type=range]::-webkit-slider-thumb {
  box-shadow: 1px 1px 1px #000000, 0px 0px 1px #0d0d0d;
  border: 1px solid #000000;
  height: 36px;
  width: 16px;
  border-radius: 3px;
  background: #ffffff;
  cursor: pointer;
  -webkit-appearance: none;
  margin-top: -14px;
}

input[type=range]:focus::-webkit-slider-runnable-track {
  background: #0090ff;
}

input[type=range]::-moz-range-track {
  width: 100%;
  height: 8.4px;
  cursor: pointer;
  animate: 0.2s;
  box-shadow: 1px 1px 1px #000000, 0px 0px 1px #0d0d0d;
  background: #3071a9;
  border-radius: 1.3px;
  border: 0.2px solid #010101;
}

input[type=range]::-moz-range-thumb {
  box-shadow: 1px 1px 1px #000000, 0px 0px 1px #0d0d0d;
  border: 1px solid #000000;
  height: 36px;
  width: 16px;
  border-radius: 3px;
  background: #ffffff;
  cursor: pointer;
}

input[type=range]::-ms-track {
  width: 100%;
  height: 8.4px;
  cursor: pointer;
  animate: 0.2s;
  background: transparent;
  border-color: transparent;
  border-width: 16px 0;
  color: transparent;
}

input[type=range]::-ms-fill-lower {
  background: #2a6495;
  border: 0.2px solid #010101;
  border-radius: 2.6px;
  box-shadow: 1px 1px 1px #000000, 0px 0px 1px #0d0d0d;
}

input[type=range]::-ms-fill-upper {
  background: #3071a9;
  border: 0.2px solid #010101;
  border-radius: 2.6px;
  box-shadow: 1px 1px 1px #000000, 0px 0px 1px #0d0d0d;
}

input[type=range]::-ms-thumb {
  box-shadow: 1px 1px 1px #000000, 0px 0px 1px #0d0d0d;
  border: 1px solid #000000;
  height: 36px;
  width: 16px;
  border-radius: 3px;
  background: #ffffff;
  cursor: pointer;
}

input[type=range]:focus::-ms-fill-lower {
  background: #3071a9;
}

input[type=range]:focus::-ms-fill-upper {
  background: #367ebd;
}
!

JS 

              
                let slider = document.getElementById('slider'),
    b = document.body;

slider.oninput = function () {
  b.style.fontSize = this.value + 'px';
  op.innerHTML = this.value + 'px';
}
!
999px

Console