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HTML

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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>

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CSS

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CSS Base

About CSS Base

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HTML 

              
                h1.text-center Super awesome flex grid

h2.text-center By default everything is just equally spaced out
.flex-container
  .flex-item.box 1
  .flex-item.box 2
  .flex-item.box 3
  .flex-item.box 4
  .flex-item.box 5

h2.text-center Add class of .flex-wrap and it can break to multiple lines
.flex-container.flex-wrap
  .flex-item.flex-1-3.box .flex-1-3
  .flex-item.flex-1-3.box .flex-1-3
  .flex-item.flex-1-3.box .flex-1-3
  .flex-item.flex-2-3.box .flex-2-3
  .flex-item.flex-1-3.box .flex-1-3

h2.text-center Goes up to a 12 column grid
.flex-container.flex-wrap
  - var n = 1;
  while n <= 12
    .flex-item.flex-1-12.box=n++

h2.text-center anything .flex-1-1 to .flex-12-12 works
.flex-container.flex-wrap
  .flex-item.flex-2-12.box .flex-2-12
  .flex-item.flex-3-12.box .flex-3-12
  .flex-item.flex-4-12.box .flex-4-12
  .flex-item.flex-1-12.box .flex-1-12
  .flex-item.flex-1-12.box .flex-1-12
  .flex-item.flex-1-6.box .flex-1-6
  .flex-item.flex-2-6.box .flex-2-6
  .flex-item.flex-3-6.box .flex-3-6
  .flex-item.flex-1-2.box .flex-1-2
  .flex-item.flex-1-4.box .flex-1-4
  .flex-item.flex-1-4.box .flex-1-4
  .flex-item.flex-3-12.box .flex-3-12
  .flex-item.flex-9-12.box .flex-9-12
  
h2.text-center 
  | add .row to any wrapper to make max-width of 1400px (configurable with variable)
.flex-container.flex-wrap.row
  - var n = 1;
  while n <= 12
    .flex-item.flex-1-12.box=n++
    
h2.text-center can add flex-1-x classes to wrapper to change width of items (this is flex-1-2)
.flex-container.flex-wrap.flex-1-2
  .flex-item.box 
  .flex-item.box
  .flex-item.box
  .flex-item.box
  
h2.text-center can add flex-1-x classes to wrapper to change width of items (this is flex-1-5)
.flex-container.flex-wrap.flex-1-5
  .flex-item.box 
  .flex-item.box
  .flex-item.box
  .flex-item.box
  .flex-item.box
  .flex-item.box
  .flex-item.box
  
h2.text-center can handle responsive classes based on variable breakpoints
h3.text-center (sm-flex-1-1, md-flex-1-2, lg-flex-1-3, xl-flex-1-4, jumbo-flex-1-5)
.flex-container.flex-wrap.flex-1-1.sm-flex-1-1.md-flex-1-2.lg-flex-1-3.xl-flex-1-4.jumbo-flex-1-5
  .flex-item.box 
  .flex-item.box
  .flex-item.box
  .flex-item.box
  .flex-item.box
!

CSS 

              
                html {
  box-sizing: border-box;
  font-size: 16px;

  * {
    box-sizing: inherit;
  }
}

.box {
  width: 100px;
  height: 100px;
  color: white;
  text-shadow: 1px 1px 4px rgba(0, 0, 0, 0.6);
  font-size: 2rem;
  padding: 10px;
  display: flex;
  justify-content: center;
  align-items: center;
}

.text-center {
  text-align: center;
}

$default: 0px;
$sm: 600px;
$md: 900px;
$lg: 1200px;
$xl: 1440px;
$jumbo: 1920px;
$row-width: 1400px;
$inner-row-width: 800px;
$gutter-width: 15px;

.row {
  margin: 0 auto;
  max-width: $row-width;
  padding: 0 $gutter-width;

  @media (min-width: $row-width + $gutter-width) {
    padding: 0;
  }
}

.row-full {
  left: 50%;
  position: relative;
  transform: translateX(-50%);
  width: 100vw;
}

.inner-row {
  margin: 0 auto;
  max-width: $inner-row-width;
  padding: 0 $gutter-width;

  @media (min-width: $inner-row-width + $gutter-width) {
    padding: 0;
  }
}

// I cannot figure out how to get around defining what breakpoints exist for the grid here & in variables.scss
$breakpoints: (
  default: $default,
  sm: $sm,
  md: $md,
  lg: $lg,
  xl: $xl,
  jumbo: $jumbo
);

@each $key, $breakpoint in $breakpoints {
  $beginning: '';
  @if $key != default {
    $beginning: "#{$key}-"; // sass-lint:disable-line space-around-operator
  }

  @media (min-width: $breakpoint) {
    .#{$beginning}flex-container {
      display: flex;
    }

    .#{$beginning}flex-wrap {
      flex-wrap: wrap;
    }

    .#{$beginning}flex-nowrap {
      flex-wrap: nowrap;
    }

    .#{$beginning}flex-wrap-reverse {
      flex-wrap: wrap-reverse;
    }

    .#{$beginning}flex-row,
    .#{$beginning}flex-dir-row {
      flex-direction: row;
    }

    .#{$beginning}flex-dir-row-reverse {
      flex-direction: row-reverse;
    }

    .#{$beginning}flex-column,
    .#{$beginning}flex-dir-column {
      flex-direction: column;
    }

    .#{$beginning}flex-dir-column-reverse {
      flex-direction: column-reverse;
    }

    .#{$beginning}flex-jc-flex-start {
      justify-content: flex-start;
    }

    .#{$beginning}flex-jc-flex-end {
      justify-content: flex-end;
    }

    .#{$beginning}flex-jc-center {
      justify-content: center;
    }

    .#{$beginning}flex-jc-space-between {
      justify-content: space-between;
    }

    .#{$beginning}flex-jc-space-around {
      justify-content: space-around;
    }

    .#{$beginning}flex-jc-space-evenly {
      justify-content: space-evenly;
    }

    .#{$beginning}flex-ai-flex-start {
      align-items: flex-start;
    }

    .#{$beginning}flex-ai-flex-end {
      align-items: flex-end;
    }

    .#{$beginning}flex-ai-center {
      align-items: center;
    }

    .#{$beginning}flex-ai-stretch {
      align-items: stretch;
    }

    .#{$beginning}flex-ai-baseline {
      align-items: baseline;
    }

    .#{$beginning}flex-ac-flex-start {
      align-content: flex-start;
    }

    .#{$beginning}flex-ac-flex-end {
      align-content: flex-end;
    }

    .#{$beginning}flex-ac-center {
      align-content: center;
    }

    .#{$beginning}flex-ac-stretch {
      align-content: stretch;
    }

    .#{$beginning}flex-ac-space-between {
      align-content: space-between;
    }

    .#{$beginning}flex-ac-space-around {
      align-content: space-around;
    }

    .#{$beginning}flex-as-flex-start {
      align-self: flex-start;
    }

    .#{$beginning}flex-as-flex-end {
      align-self: flex-end;
    }

    .#{$beginning}flex-as-center {
      align-self: center;
    }

    .#{$beginning}flex-as-baseline {
      align-self: baseline;
    }

    .#{$beginning}flex-as-stretch {
      align-self: stretch;
    }

    .flex-container > *,
    .flex-item {
      flex: 1 1 100%;
      // on ie11 there is a bug with flex item's padding and box-sizing: border-box, but forcing 0 padding
      // caused other issues like forcing people to use !important tags just to add some padding to flex items.
      padding: 0;

      &.#{$beginning}flex-1-12,
      .#{$beginning}flex-1-12 & {
        flex: 1 1 8.333%;
      }

      &.#{$beginning}flex-1-11,
      .#{$beginning}flex-1-11 & {
        flex: 1 1 9.09%;
      }

      &.#{$beginning}flex-1-10,
      .#{$beginning}flex-1-10 & {
        flex: 1 1 10%;
      }

      &.#{$beginning}flex-1-9,
      .#{$beginning}flex-1-9 & {
        flex: 1 1 11.111%;
      }

      &.#{$beginning}flex-1-8,
      .#{$beginning}flex-1-8 & {
        flex: 1 1 12.5%;
      }

      &.#{$beginning}flex-1-7,
      .#{$beginning}flex-1-7 & {
        flex: 1 1 14.285%;
      }

      &.#{$beginning}flex-1-6,
      &.#{$beginning}flex-2-12,
      .#{$beginning}flex-1-6 & {
        flex: 1 1 16.666%;
      }

      &.#{$beginning}flex-2-11 {
        flex: 1 1 18.181%;
      }

      &.#{$beginning}flex-1-5,
      &.#{$beginning}flex-2-10,
      .#{$beginning}flex-1-5 & {
        flex: 1 1 20%;
      }

      &.#{$beginning}flex-2-9 {
        flex: 1 1 22.222%;
      }

      &.#{$beginning}flex-1-4,
      &.#{$beginning}flex-2-8,
      &.#{$beginning}flex-3-12,
      .#{$beginning}flex-1-4 & {
        flex: 1 1 25%;
      }

      &.#{$beginning}flex-3-11 {
        flex: 1 1 27.272%;
      }

      &.#{$beginning}flex-2-7 {
        flex: 1 1 28.571%;
      }

      &.#{$beginning}flex-3-10 {
        flex: 1 1 30%;
      }

      &.#{$beginning}flex-1-3,
      &.#{$beginning}flex-2-6,
      &.#{$beginning}flex-3-9,
      &.#{$beginning}flex-4-12,
      .#{$beginning}flex-1-3 & {
        flex: 1 1 33.333%;
      }

      &.#{$beginning}flex-4-11 {
        flex: 1 1 36.363%;
      }

      &.#{$beginning}flex-3-8 {
        flex: 1 1 37.5%;
      }

      &.#{$beginning}flex-2-5,
      &.#{$beginning}flex-4-10 {
        flex: 1 1 40%;
      }

      &.#{$beginning}flex-5-12 {
        flex: 1 1 41.666%;
      }

      &.#{$beginning}flex-3-7 {
        flex: 1 1 42.857%;
      }

      &.#{$beginning}flex-4-9 {
        flex: 1 1 44.444%;
      }

      &.#{$beginning}flex-5-11 {
        flex: 1 1 45.454%;
      }

      &.#{$beginning}flex-1-2,
      &.#{$beginning}flex-2-4,
      &.#{$beginning}flex-3-6,
      &.#{$beginning}flex-4-8,
      &.#{$beginning}flex-5-10,
      &.#{$beginning}flex-6-12,
      .#{$beginning}flex-1-2 & {
        flex: 1 1 50%;
      }

      &.#{$beginning}flex-6-11 {
        flex: 1 1 54.545%;
      }

      &.#{$beginning}flex-5-9 {
        flex: 1 1 55.555%;
      }

      &.#{$beginning}flex-4-7 {
        flex: 1 1 57.142%;
      }

      &.#{$beginning}flex-7-12 {
        flex: 1 1 58.333%;
      }

      &.#{$beginning}flex-6-10 {
        flex: 1 1 60%;
      }

      &.#{$beginning}flex-5-8 {
        flex: 1 1 62.5%;
      }

      &.#{$beginning}flex-7-11 {
        flex: 1 1 63.636%;
      }

      &.#{$beginning}flex-2-3,
      &.#{$beginning}flex-3-5,
      &.#{$beginning}flex-4-6,
      &.#{$beginning}flex-6-9,
      &.#{$beginning}flex-8-12 {
        flex: 1 1 66.666%;
      }

      &.#{$beginning}flex-7-10 {
        flex: 1 1 70%;
      }

      &.#{$beginning}flex-5-7 {
        flex: 1 1 71.428%;
      }

      &.#{$beginning}flex-8-11 {
        flex: 1 1 72.727%;
      }

      &.#{$beginning}flex-3-4,
      &.#{$beginning}flex-6-8,
      &.#{$beginning}flex-9-12 {
        flex: 1 1 75%;
      }

      &.#{$beginning}flex-7-9 {
        flex: 1 1 77.777%;
      }

      &.#{$beginning}flex-4-5,
      &.#{$beginning}flex-8-10 {
        flex: 1 1 80%;
      }

      &.#{$beginning}flex-9-11 {
        flex: 1 1 81.818%;
      }

      &.#{$beginning}flex-5-6,
      &.#{$beginning}flex-10-12 {
        flex: 1 1 83.333%;
      }

      &.#{$beginning}flex-6-7 {
        flex: 1 1 85.714%;
      }

      &.#{$beginning}flex-7-8 {
        flex: 1 1 87.5%;
      }

      &.#{$beginning}flex-8-9 {
        flex: 1 1 88.888%;
      }

      &.#{$beginning}flex-9-10 {
        flex: 1 1 90%;
      }

      &.#{$beginning}flex-10-11 {
        flex: 1 1 90.909%;
      }

      &.#{$beginning}flex-11-12 {
        flex: 1 1 91.666%;
      }

      &.#{$beginning}flex-1-1,
      &.#{$beginning}flex-2-2,
      &.#{$beginning}flex-3-3,
      &.#{$beginning}flex-4-4,
      &.#{$beginning}flex-5-5,
      &.#{$beginning}flex-6-6,
      &.#{$beginning}flex-7-7,
      &.#{$beginning}flex-8-8,
      &.#{$beginning}flex-9-9,
      &.#{$beginning}flex-10-10,
      &.#{$beginning}flex-11-11,
      &.#{$beginning}flex-12-12 {
        flex: 1 1 100%;
      }

      &.#{$beginning}flex-shrink-0 {
        flex-shrink: 0;
      }
      
      &.#{$beginning}flex-grow-0 {
        flex-grow: 0;
      }

      &.#{$beginning}flex-grow-2 {
        flex-grow: 2;
      }

      &.#{$beginning}flex-grow-3 {
        flex-grow: 3;
      }

      &.#{$beginning}flex-grow-4 {
        flex-grow: 4;
      }

      &.#{$beginning}flex-grow-5 {
        flex-grow: 5;
      }
    }
  }
}
!

JS 

              
                const randomColor = () => '#'+Math.floor(Math.random()*16777215).toString(16);

let boxes = document.querySelectorAll('.box');

for (let i=0; i<boxes.length; i++) {
  boxes[i].style.background = randomColor();
}
!
999px

Console