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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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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-.

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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 

              
                .panel
  h2 flex-direction: row ↔
  .legend
    p.ac align-content ↕
    p.jc justify-content ↔
    p.ai align-items ↕
    p.ji.inactive justify-items ↔

  .container#row

    .row-helper
      .helper
        .item
          p align-self ↕
          p.inactive justify-self ↔
      .helper
        .item
          p align-self ↕
          p.inactive justify-self  ↔
      .helper
        .item
          p align-self ↕
          p.inactive justify-self ↔
    .row-helper
      .helper
        .item
          p align-self ↕
          p.inactive justify-self ↔
      .helper
        .item
          p align-self ↕
          p.inactive justify-self ↔

.panel
  h2 flex-direction: column ↕

  .legend
    p.ac align-content ↔
    p.jc justify-content ↕
    p.ai align-items ↔
    p.ji.inactive justify-items ↕

  .container#column

    .column-helper
      .helper
        .item
          p align-self ↔
          p.inactive justify-self ↕
      .helper
        .item
          p align-self ↔
          p.inactive justify-self  ↕
      .helper 
        .item
          p align-self ↔
          p.inactive justify-self  ↕   
    .column-helper
      .helper
        .item
          p align-self ↔
          p.inactive justify-self ↕
      .helper
        .item
          p align-self ↔
          p.inactive justify-self ↕
!

CSS 

              
                panel:first-child {
  margin-right: 20px;
  width: 350px;
}

.container {
  width: 300px;
  height: 300px;
  border: 1px solid green;
}
#column, #row {
  display: grid;
  grid: repeat(2, 100px) / repeat(3, 100px);
  grid-gap: 2px;
}

#column *, #row * {
  animation-play-state: paused;
}

#row .helper + .helper {
  margin-left: 2px;
}
.row-helper:first-child .helper:nth-child(1) { animation: row-item1 6s ease-in-out infinite; }
.row-helper:first-child .helper:nth-child(2) { animation: row-item2 6s ease-in-out infinite; }
.row-helper:first-child .helper:nth-child(3) { animation: row-item3 6s ease-in-out infinite; }
.row-helper:last-child .helper:nth-child(1) { animation: row-item4 6s ease-in-out infinite; }
.row-helper:last-child .helper:nth-child(2) { animation: row-item5 6s ease-in-out infinite; }


#column .helper + .helper {
  margin-top: 2px;
}
.column-helper:first-child .helper:nth-child(1) { animation: col-item1 6s ease-in-out infinite; }
.column-helper:first-child .helper:nth-child(2) { animation: col-item2 6s ease-in-out infinite; }
.column-helper:first-child .helper:nth-child(3) { animation: col-item3 6s ease-in-out infinite; }
.column-helper:last-child .helper:nth-child(1) { animation: col-item4 6s ease-in-out infinite; }
.column-helper:last-child .helper:nth-child(2) { animation: col-item5 6s ease-in-out infinite; }

.item {
  background: rgba(255,160,160,.5);
  border: 1px solid rgba(255,0,0,.5);
  height: 80px;
  width: 80px;
  .row-helper & {
    animation: item-align-r 4s ease-in-out infinite;
  }
  .column-helper & {
    animation: item-align-c 4s ease-in-out infinite;
  }
  .helper:nth-last-child(2) & {
    height: 96px;
    width: 96px;
    animation: none;
  }
  .row-helper:first-child :first-child & {
    width: 90px;
  }
  .column-helper:first-child :first-child & {
    height: 90px;
  }
  p {
    margin: .5em 2px;
    transform: scale(.8);
    transform-origin: 0 50%;
    transition: transform .6s;
    cursor: pointer;
    &:hover {
      transform: none;
    }
  }
  #column & + & {
    margin-top: 2px;
  }
  #row & + & {
    margin-left: 2px;
  }
}

.row-helper {
  display: flex;
  grid-column: 1 / -1;
  grid-row: 1;
  width: 300px;
  background: rgba(0,255,0,.2) 50%;
  border: dashed rgba(0,255,0,.5);
  border-width: 1px 0;
  animation: row1 6s ease-in-out infinite;
}
.row-helper + .row-helper {
  grid-row: 2;
  animation: row2 6s ease-in-out infinite;
}

.column-helper {
  display: flex;
  flex-flow: column;
  height: 300px;
  background: rgba(0,0,255,.2);
  border: dashed rgba(0,0,255,.5);
  border-width: 0 1px;
  animation: col1 6s ease-in-out infinite;
}
.column-helper + .column-helper {
  grid-column: 2;
  animation: col2 6s ease-in-out infinite;
}

.jc + .container:not(#false) .helper,
.ac + #row .row-helper,
.ac + #column .column-helper,
.ai + #row .item,
.ai + #column .item,
#row .as .item,
#column .as .item
{
  animation-play-state: running;
}

body {
  display: flex;
  flex-flow: row wrap;
  justify-content: space-evenly;
  font: 15px/1 sans-serif;
}

.legend {
  display: grid;
  grid: repeat(2, auto) / repeat(2, 170px);
  width: 340px;
  grid-auto-flow: column;
  margin: 0 auto 1em;
  font-size: 20px;
  
  p {
    margin: .5em;
    transform: scale(.8);
    transform-origin: 0 50%;
    transition: transform .6s;
    cursor: pointer;
  }
}
.ac .ac,
.jc .jc,
.ai .ai,
.ji .ji {
  transform: none;
}

@keyframes col1 {
  0%, 100% { transform: translateX(0); }
  17% { transform: translateX(49px); }
  33% { transform: translateX(98px); }
  50% { transform: translateX(33px); }
  67% { transform: translateX(23px); }
  83% { transform: translateX(0); }
}

@keyframes col2 {
  0%, 100% { transform: translateX(0); }
  17% { transform: translateX(49px); }
  33% { transform: translateX(98px); }
  50% { transform: translateX(65px); }
  67% { transform: translateX(75px); }
  83% { transform: translateX(98px); }
}

@keyframes row1 {
  0%, 100% { transform: translateY(0); }
  17% { transform: translateY(49px); }
  33% { transform: translateY(98px); }
  50% { transform: translateY(33px); }
  67% { transform: translateY(23px); }
  83% { transform: translateY(0); }
}

@keyframes row2 {
  0%, 100% { transform: translateY(0); }
  17% { transform: translateY(49px); }
  33% { transform: translateY(98px); }
  50% { transform: translateY(65px); }
  67% { transform: translateY(75px); }
  83% { transform: translateY(98px); }
}

@keyframes item-align-c {
  0%, 100% { transform: translateX(0); }
  33% { transform: translateX(8px); }
  67% { transform: translateX(17px); }
}

@keyframes item-align-r {
  0%, 100% { transform: translateY(0); }
  33% { transform: translateY(8px); }
  67% { transform: translateY(17px); }
}

@keyframes row-item1 {
  0%, 100% { transform: translateX(0); }
  17% { transform: translateX(12px); }
  33% { transform: translateX(24px); }
  50% { transform: translateX(8px); }
  67% { transform: translateX(6px); }
  83% { transform: translateX(0); }
}
@keyframes row-item2 {
  0%, 100% { transform: translateX(0); }
  17% { transform: translateX(12px); }
  33% { transform: translateX(24px); }
  50% { transform: translateX(12px); }
  67% { transform: translateX(12px); }
  83% { transform: translateX(12px); }
}
@keyframes row-item3 {
  0%, 100% { transform: translateX(0); }
  17% { transform: translateX(12px); }
  33% { transform: translateX(24px); }
  50% { transform: translateX(16px); }
  67% { transform: translateX(18px); }
  83% { transform: translateX(24px); }
}
@keyframes row-item4 {
  0%, 100% { transform: translateX(0); }
  17% { transform: translateX(59px); }
  33% { transform: translateX(118px); }
  50% { transform: translateX(39px); }
  67% { transform: translateX(29px); }
  83% { transform: translateX(0); }
}
@keyframes row-item5 {
  0%, 100% { transform: translateX(0); }
  17% { transform: translateX(59px); }
  33% { transform: translateX(118px); }
  50% { transform: translateX(79px); }
  67% { transform: translateX(89px); }
  83% { transform: translateX(118px); }
}


@keyframes col-item1 {
  0%, 100% { transform: translateY(0); }
  17% { transform: translateY(12px); }
  33% { transform: translateY(24px); }
  50% { transform: translateY(8px); }
  67% { transform: translateY(6px); }
  83% { transform: translateY(0); }
}
@keyframes col-item2 {
  0%, 100% { transform: translateY(0); }
  17% { transform: translateY(12px); }
  33% { transform: translateY(24px); }
  50% { transform: translateY(12px); }
  67% { transform: translateY(12px); }
  83% { transform: translateY(12px); }
}
@keyframes col-item3 {
  0%, 100% { transform: translateY(0); }
  17% { transform: translateY(12px); }
  33% { transform: translateY(24px); }
  50% { transform: translateY(16px); }
  67% { transform: translateY(18px); }
  83% { transform: translateY(24px); }
}
@keyframes col-item4 {
  0%, 100% { transform: translateY(0); }
  17% { transform: translateY(59px); }
  33% { transform: translateY(118px); }
  50% { transform: translateY(39px); }
  67% { transform: translateY(29px); }
  83% { transform: translateY(0); }
}
@keyframes col-item5 {
  0%, 100% { transform: translateY(0); }
  17% { transform: translateY(59px); }
  33% { transform: translateY(118px); }
  50% { transform: translateY(79px); }
  67% { transform: translateY(89px); }
  83% { transform: translateY(118px); }
}

p.inactive  {
  color: rgba(0,0,0,.3);
}
!

JS 

              
                
document.querySelectorAll('.legend').forEach(elem => elem.onmouseover = function(e) {
  let target = e.target;
  if (target.className == 'legend') return;
  this.className = 'legend ' + target.className;
});

document.querySelectorAll('.legend').forEach(elem => elem.onmouseout = function(e) {
  this.className = 'legend';
});

document.querySelectorAll('.container').forEach(elem => elem.onmouseover = function(e) {
  let target = e.target;
  if (target.parentNode.className != 'item') return;
  if (target.nextElementSibling) {
    target.closest('.helper').className = 'helper as';
  }
});

document.querySelectorAll('.container').forEach(elem => elem.onmouseout = function(e) {
  let target = e.target;
  if (target.parentNode.className != 'item') return;
  target.closest('.helper').className = 'helper';
});
!
999px

Console