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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

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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).

Add External Stylesheets/Pens

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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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HTML 

              
                <main>
  <h1>Vertical &amp; horizontal centring*</h1>


  <section class=container-flexbox>
    <div class=centered-flexbox>

      <figure>
      <figcaption>IE10+ (preferred method)</figcaption>
        <pre><code class=language-css spellcheck=false contenteditable>.container-flexbox { 
  min-height: 100vh; /* Unit &amp; size irrelevant */
  display: flex; /* Collapses box */
  align-items: center;
  justify-content: center;
}
.centered-flexbox {
}</code></pre>
      </figure>

    </div>
  </section>


  <section class=container-grid>
    <div class=centered-grid>

      <figure>
      <figcaption>Grid method</figcaption>
        <pre><code class=language-css spellcheck=false contenteditable>.container-grid {
  min-height: 100vh; /* Unit &amp; size irrelevant */
  display: grid;
  place-content: center;
}
.centered-grid {
}</code></pre>
      </figure>

    </div>
  </section>



  <section class=container-noFixedHeight>
    <div class=centered-noFixedHeight>

      <figure>
      <figcaption>IE9+</figcaption>
        <pre><code class=language-css spellcheck=false contenteditable>.container-noFixedHeight {
  position: relative;
  min-height: 100vh; /* Unit &amp; size irrevelant */
}
.centered-noFixedHeight {
  position: absolute; /* Collapses box */
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}</code></pre>
      </figure>

    </div>
  </section>



  <section class=container-float>

    <div class=centered-float>
      <figcaption>IE9+</figcaption>
      <figure>
        <pre><code class=language-css spellcheck=false contenteditable>.container-float {
  height: 100vh; /* Unit irrelevant */
}
.container-float::after {
  content: "";
  display: table;
  clear: both;
}
.centered-float {
  position: relative;
  float: left; /* Collapses box */
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}</code></pre>
      </figure>
    </div>

  </section>



  <section class=container-ellipsis>
    <div class=centered-ellipsis>
      <span class=centered-ellipsis-span>
        Centre?
      </span>
    </div>
  </section>



  <section class=container-credits>
    <div class=centered-credits>

      <p>Further variants: <a class=lnk rel=noopener target=_blank title="[new window]" href="https://web.dev/centering-in-css/" target=_blank title="[new window]">Centering in CSS</a></p>

      <p>More options and variants: <a class=lnk rel=noopener target=_blank title="[new window]" href="https://www.smashingmagazine.com/2013/08/absolute-horizontal-vertical-centering-css/" target=_blank title="[new window]">Absolute horizontal vertical centering CSS</a></p>

      <p>Modern CSS: <a class=lnk rel=noopener target=_blank title="[new window]" href="https://moderncss.dev/resource-the-complete-guide-to-centering-in-css/">Complete Guide to Centering in CSS</a></p>

      <p>CSS Tricks: <a class=lnk rel=noopener target=_blank title="[new window]" href="https://css-tricks.com/centering-the-newest-coolest-way-vs-the-oldest-coolest-way/">Centering - newest vs oldest</a></p>

      <p>Photos: <a class=lnk rel=noopener target=_blank title="[new window]" href="https://unsplash.com/">Unsplash</a>

      <p>*Brit eh?</p>

    </div>
  </section>
</main>

[[[https://codepen.io/2kool2/pen/mKeeGM]]]
!

CSS 

              
                /* Generic styles */
body {
  font-family: sans-serif;
  line-height: 1.5;
  margin: 0;
  background-color: #1a1a1a;
  color: #fff;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}
main {
  height: 100vh;
  overflow-y: scroll;
  scroll-snap-type: y mandatory;
}
h1 {
  font-weight: 100;
  position: fixed;
  color: #000;
  background-color: rgba(255,255,255, .8);
  top: 1rem;
  left: 50%;
  transform: translate(-50%, 0);
  padding: .5rem 1rem;
  width: 50%;
  min-width: 20rem;
  margin: 0 auto;
  text-align: center;
  line-height: 1.2;
}
p {margin: 1rem; max-width: 34rem;}

figure {margin: 0;}
figcaption {
  background-color: #1a1a1a;
  text-align: center;
  padding: .25rem;
  font-weight: 700;
}
a {
  color: #fff !important;
  text-decoration: none;
  border-bottom: 1px solid rgba(255,255,255, .5);
}

[class^="container-"] {
  scroll-snap-align: start;
}

[class^="centered-"] {
  
  /* Optional: Keep inside viewport */
  max-width: calc(100% - 2rem);
  
  /* OPTIONAL: */
  max-height: calc(100vh - 4rem);

  /* Not required but plays nice */
  transition: all .3s ease-out;
}
[class^="centered-"]:hover,
[class^="centered-"]:focus-within {
  box-shadow: 0 0 .5rem .25rem rgba(0,0,0, .75);
}


/* Required centring styles */


 /* Preferred but doesn't work at all in IE9 */
.container-flexbox {
  min-height: 100vh; /* Unit & size irrevelant */
  display: flex;
  align-items: center;
  justify-content: center;
}
.centered-flexbox {
}


/* OR */


 /* Doesn't work at all in IE9 */
.container-grid {
  min-height: 100vh; /* Unit & size irrevelant */
  display: grid;
  place-content: center;
}
.centered-grid {
/*   margin: 0 auto; */
}


/* OR */


.container-noFixedHeight {
  position: relative;
  min-height: 100vh; /* Unit & size irrevelant */
}
.centered-noFixedHeight {
  position: absolute; /* Collapses box */
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}


/* OR */


.container-float {
  height: 100vh; /* Unit irrevelant */
  /* padding: 1rem; /* Optional */
}
.container-float::after {
  content: "";
  display: table;
  clear: both;
}
.centered-float {
  position: relative;
  float: left; /* Collapses box */
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}


/* OR */


.container-ellipsis {
  position: relative;
  height: 100vh; /* Unit irrevelant */
}
.centered-ellipsis,
.centered-ellipsis-span {
  position: absolute; /* Collapses box */
  max-width: 50vmin;
  max-height: 50vmin;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}
.centered-ellipsis {
  /* Sets this div to 50% sq */
  bottom: 0;
  right: 0;
}
.centered-ellipsis-span {
}


/* Credits section */
.container-credits {
  height: 100vh; /* Unit irrevelant */
  display: flex;
  align-items: center;
  justify-content: center;
}


/* Pretty-pretties - Not required but sweet */
/* Images unsplash.com */

.container-float {
  background-position: center bottom;
  background-size: cover;
  /* Dmitri Popov */
  background-image: url(https://images.unsplash.com/photo-1497514935393-24b153421536?w=1920);
}
.container-noFixedHeight {
  background-position: center;
  background-size: cover;
  /* Michał Grosicki */
  background-image: url(https://images.unsplash.com/photo-1489343511429-5482f78c15cf?w=1920);
}
.container-absolute {
  background-position: center;
  background-size: cover;
  /* Michał Grosicki */
  background-image: url(https://images.unsplash.com/photo-1494187042164-c3d951f93e5c?w=1920);
}
.container-flexbox {
  background-position: center bottom;
  background-size: cover;
  /* James Padolsey */
  background-image: url(https://images.unsplash.com/photo-1476958526483-36efcaa80b1b?w=1920);
}
.container-grid {
  background-position: center top;
  background-size: cover;
  /* Wolfgang Hasselmann */
  background-image: url(https://unsplash.com/photos/mZabJ9h8_jc/download?force=true&w=1920);
}


.container-ellipsis {
  background-position: center;
  background-size: cover;
  /* Jeremy Thomas */
  background-image: url(https://unsplash.com/photos/E0AHdsENmDg/download?force=true&w=1920);
}
.centered-ellipsis {
  border-radius: 50%;
  box-shadow: inset 0 2.5rem 5rem 1.5rem rgba(0,0,0, .8), 0 0 1rem .125rem #000;
  background: linear-gradient(to bottom, #342a29 0%,#b4300a 9%,#ffc102 30%,#edddd7 44%,#e8ebf2 50%,#edddd7 56%,#ffc102 70%,#b4300a 87%,#342a29 100%);
}
.centered-ellipsis:hover {
  box-shadow: inset 0 2.5rem 5rem 1.5rem rgba(0,0,0, .8), 0 0 1rem .125rem #000;
}
.centered-ellipsis-span {
  background-color: #000;
  padding: 1rem;
  font-weight: 700;
  border-radius: 50%;
  box-shadow: 0 0 1rem 2rem #b4300a;
  box-shadow: 0 0 3vmin 4vmin #b4300a;
}


.container-credits {
  padding: 1rem;
  height: calc(100vh - 2rem);
  background-position: center bottom;
  background-size: cover;
  background-image: url(https://images.unsplash.com/photo-1431576901776-e539bd916ba2?w=1920);
}
.centered-credits {
  background-color: rgba(0,0,0, .85);
}
.centered-credits:hover {
  background-color: rgba(0,0,0, 1);
}
!

JS 

              
                // Prism used for code highlighting
// Force dark mode
document.body.setAttribute('data-lightmode', 'dark');
!
999px

Console