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HTML

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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

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JavaScript

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Babel includes JSX processing.

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HTML 

              
                <fieldset id="wrap" style="view-transition-name: wrap">
  <legend>Flex Wrap</legend>
  
  <label>
    Wrap
    <input type="radio" name="wrap-controls" value="wrap" checked>
  </label>
  
  <label>
    No Wrap
    <input type="radio" name="wrap-controls" value="nowrap">
  </label>
</fieldset>

<fieldset id="direction" style="view-transition-name: direction">
  <legend>Flex Direction</legend>
  
  <label>
    Row
    <input type="radio" name="direction-controls" value="row" checked>
  </label>
  
  <label>
    Row Reverse
    <input type="radio" name="direction-controls" value="row-reverse">
  </label>
  
  <label>
    Column
    <input type="radio" name="direction-controls" value="column">
  </label>
  
  <label>
    Column Reverse
    <input type="radio" name="direction-controls" value="column-reverse">
  </label>
</fieldset>

<fieldset id="alignItems" style="view-transition-name: align-items">
  <legend>Align Items</legend>
  
  <label>
    Start
    <input type="radio" name="align-items-controls" value="start" checked>
  </label>
  
  <label>
    Center
    <input type="radio" name="align-items-controls" value="center">
  </label>
  
  <label>
    End
    <input type="radio" name="align-items-controls" value="end">
  </label>
  
  <label>
    Stretch
    <input type="radio" name="align-items-controls" value="stretch">
  </label>
</fieldset>

<fieldset id="alignContent" style="view-transition-name: align-content">
  <legend>Align Content</legend>
  
  <label>
    Start
    <input type="radio" name="align-content-controls" value="start" checked>
  </label>
  
  <label>
    Center
    <input type="radio" name="align-content-controls" value="center">
  </label>
  
  <label>
    End
    <input type="radio" name="align-content-controls" value="end">
  </label>
  
  <label>
    Space Between
    <input type="radio" name="align-content-controls" value="space-between">
  </label>
  
  <label>
    Space Around
    <input type="radio" name="align-content-controls" value="space-around">
  </label>
  
  <label>
    Space Evenly
    <input type="radio" name="align-content-controls" value="space-evenly">
  </label>
  
<!--   <label>
    Stretch
    <input type="radio" name="align-content-controls" value="stretch">
  </label> -->
</fieldset>

<fieldset id="justifyContent" style="view-transition-name: justify-content">
  <legend>Justify Content</legend>
  
  <label>
    Start
    <input type="radio" name="justify-content-controls" value="start" checked>
  </label>
  
  <label>
    Center
    <input type="radio" name="justify-content-controls" value="center">
  </label>
  
  <label>
    End
    <input type="radio" name="justify-content-controls" value="end">
  </label>
  
  <label>
    Space Between
    <input type="radio" name="justify-content-controls" value="space-between">
  </label>
  
  <label>
    Space Around
    <input type="radio" name="justify-content-controls" value="space-around">
  </label>
  
  <label>
    Space Evenly
    <input type="radio" name="justify-content-controls" value="space-evenly">
  </label>
  
  <label>
    Stretch
    <input type="radio" name="justify-content-controls" value="stretch">
  </label>
</fieldset>

<div class="flex" id="demo">
  <div class="item" style="view-transition-name: item-1">1</div>
  <div class="item" style="view-transition-name: item-2">2</div>
  <div class="item" style="view-transition-name: item-3">3</div>
  <div class="item" style="view-transition-name: item-4">4</div>
  <div class="item" style="view-transition-name: item-5">5</div>
  <div class="item" style="view-transition-name: item-6">6</div>
  <div class="item" style="view-transition-name: item-7">7</div>
</div>
!

CSS 

              
                @layer demo {
  .flex {
    display: flex;
    gap: 1rem;
    flex-wrap: wrap;
    align-items: start;
    justify-content: start;
    
    border: 1px solid Highlight;
    padding: 1rem;
    
    inline-size: min(80vw, 1024px);
    block-size: 50vh;
    margin-block-start: 5vh;
  }
  
  .item {
    color: Canvas;
    background: CanvasText;
    inline-size: min(15vw, 175px);
    aspect-ratio: 4/3;
    
    &:nth-child(3) {
      aspect-ratio: 3/4;
    }
    
    &:nth-child(6) {
      aspect-ratio: 16/9;
    }
  }
}

@layer demo.support {
  * {
    box-sizing: border-box;
    margin: 0;
  }

  html {
    block-size: 100%;
    color-scheme: dark light;
  }

  body {
    min-block-size: 100%;
    font-family: system-ui, sans-serif;

    display: grid;
    place-content: center;
    gap: 1rem;
    padding: 1rem;
  }
  
  fieldset {
    max-inline-size: max-content;
    display: flex;
    flex-wrap: wrap;
    gap: 1ch 2ch;
    border-color: #8885;
    
    > label {
      display: flex;
      gap: 1ch;
      align-items: center;
      white-space: nowrap;
    }
  }
}
!

JS 

              
                // make demo cross browser, just wont animate if they cant
function mutate(prop, val) {
  if (document.startViewTransition) {
    document.startViewTransition(() => {
      demo.style[prop] = val
    })
  }
  else {
    demo.style[prop] = val
  }
}

direction.oninput = e => {
  mutate('flexDirection', e.target.value)
}

wrap.oninput = e => {
  mutate('flexWrap', e.target.value)
}

justifyContent.oninput = e => {
  mutate('justifyContent', e.target.value)
}

alignItems.oninput = e => {
  mutate('alignItems', e.target.value)
}

alignContent.oninput = e => {
  mutate('alignContent', e.target.value)
}
!
999px

Console