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HTML 

              
                <section class="section__1">
    <h1>Yes, this is that presentation</h1>
    <p>It's about flexbox</p>
</section>

<section class="section__1">
  <h1>S1: flex-direction: row (default)</h1>
  <div class="flex ">
    <div class="item">s1 item 1</div>
    <div class="item">s1 item 2</div>
    <div class="item">s1 Some more txt in this item</div>
    <div class="item">s1 item 4</div>
  </div>
  <p>items will display as a row and line up at the start of that axis. <br>still space at the end of the flex container.</p>
</section>

<section class="section__1-b">
  <h1>S1-b: flex-direction: row but with more text</h1>
  <div class="flex ">
    <div class="item">s1 item 1</div>
    <div class="item">s1 item 2</div>
    <div class="item">This behavior is likely to be familiar to you if you have ever used Flexbox, but perhaps you have wondered how the browser is working that sizing out, as if you look in multiple modern browsers you will see that they all do the same thing</div>
  </div>
  <p>more text to these items so they will fill the container</p>
</section>



<section class="section__1-c">
  <h1>S1-c: 
    flex-grow:1 / flex-basis: auto
  </h1>
  <div class="flex">
    <div class="item">Wouldn't</div>
    <div class="item">we</div>
    <div class="item">better start farming potatoes ?</div>
  </div>
  <p>short version: flex: 1 1 auto  ==> default</p>
</section>


<section class="section__1-c">
  <h1>S1-c with more text: 
    flex-grow:0 / flex-basis: auto
  </h1>
  <div class="flex">
    <div class="item">Wouldn't</div>
    <div class="item">we</div>
    <div class="item">This behavior is likely to be familiar to you if you have ever used Flexbox, but perhaps you have wondered how the browser </div>
  </div>
  <p>No space to distribute here</p>
</section>


<section class="section__1-d">
  <h1>S1-d: Flex-basis: 0</h1>
  <div class="flex ">
    <div class="item">Wouldn't</div>
    <div class="item">we</div>
    <div class="item">This behavior is likely to be familiar to you if you have ever used Flexbox, but perhaps you have wondered how the browser </div>
  </div>
  <p>short version: flex: 1 1 0; <br>flex-basis to 0 and flex-grow to 1 <br> all of our boxes: no starting width <br>so space in  flex container is shared out evenly</p>
</section>
<hr>

<section class="section__2">
  <h1>S2 : flex-direction: column;</h1>
  <div class="flex flex-col">
    <div class="item">s2 item 1</div>
    <div class="item">s2 item 2</div>
    <div class="item">This behavior is likely to be familiar to you if you have ever used Flexbox, but perhaps you have wondered how the browser is working that sizing out, as if you look in multiple modern browsers you will see that they all do the same thing. </div>
  </div>
</section>

<section class="section__3">
  <h1>S3: flex-direction: row-reverse</h1>
  <div class="flex flex-row-reverse">
    <div class="item flex-grow">s3 item 1</div>
    <div class="item flex-grow">s3 item 2</div>
    <div class="item flex-grow">s3 item 3</div>
  </div>
  <p>Reverse the order of items in a row direction</p>
</section>


<section class="section__4">
  <h1>S4: flex-direction: col-reverse</h1>
  <div class="flex flex-col-reverse">
    <div class="item flex-grow">s4 item 1</div>
    <div class="item flex-grow">s4 item 2</div>
    <div class="item flex-grow">s4 item 3</div>
  </div>
  <p>Reverse the order of items in a column direction</p>
</section>

<hr>


<section class="section__5">
  <h1>S5 Flex-wrap but no grow</h1>
  <div  class="flex flex-wrap">
    <div class="item">Orange potato</div>
    <div class="item">Sprout tomato pea</div>
    <div class="item">Gumbo beet</div>
    <div class="item">greens corn soko</div>
    <div class="item">endive gumbo gourd.</div>
    <div class="item">Parsley shallot courgette tatsoi pea</div>
  </div>
  <p>Makes the items (flex-grow: 0) wrap to the next row</p>
</section>

<section class="section__5">
  <h1>S5 Flex-wrap with growing items</h1>
  <div  class="flex flex-wrap">
    <div class="item flex-grow">Orange potato</div>
    <div class="item flex-grow">Sprout tomato pea</div>
    <div class="item flex-grow">Gumbo beet</div>
    <div class="item flex-grow">greens corn soko</div>
    <div class="item flex-grow">endive gumbo gourd.</div>
    <div class="item flex-grow">Parsley shallot courgette tatsoi pea</div>
  </div>
  <p>Makes the items (flex-grow: 1) wrap to the next row</p>
</section>

<hr>

<section class="section__6">
  <h1>S6 Ordering items</h1>
  <div  class="flex">
    <div class="item">first item</div>
    <div class="item">second item</div>
    <div class="item">third item</div>
    <div class="item">fourth item</div>
    <div class="item">last item</div>
  </div>
  <p>Order in the flexbox container !!</p>
</section>

<hr>

<section class="section__7">
  <h1>S7 Justification: center</h1>
  <div  class="flex">
    <div class="item">first item - some more content</div>
    <div class="item">second item</div>
    <div class="item">third item</div>
    <div class="item">fourth item</div>
    <div class="item">last item</div>
  </div>
  <p>Justify: center</p>
</section>


<section class="section__7b">
  <h1>S7b Justification: space-between</h1>
  <div  class="flex justify-between">
    <div class="item">first item - some more content</div>
    <div class="item">second item</div>
    <div class="item">third item</div>
    <div class="item">fourth item</div>
    <div class="item">last item</div>
  </div>
  <p>Justify: spread the available space "between" the items</p>
</section>


<section class="section__7c">
  <h1>S7c Justification: space-around</h1>
  <div  class="flex justify-around">
    <div class="item">first item - some more content</div>
    <div class="item">second item</div>
    <div class="item">third item</div>
    <div class="item">fourth item</div>
    <div class="item">last item</div>
  </div>
  <p>Justify: spread the available space "around" the items</p>
</section>



<section class="section__8">
  <h1>S8 Practical example</h1>
  <div class="nav-wrapper border border-dashed border-blue-400 flex flex-col-reverse md:flex-col">
  <nav class="meta flex flex-col sm:flex-row justify-between items-center">
    <div class="search p-2 flex">
      Search
      <input class="searchBox-query form-control ml-4 px-4 border border-solid border-grey-500" name="query" placeholder="Zoeken" type="search">
    </div>
    <div class="languages">
      <ul class="flex flex-row">
        <li><a class="p-2" href="">Nederlands</a></li>
        <li><a class="p-2" href="">Frans</a></li>
        <li><a class="p-2" href="">Engels</a></li>
      </ul>
    </div>
  </nav>
  <header class="header flex flex-col sm:flex-row justify-between items-center bg-white p-2 sm:p4"><a href="/" class="flex items-center text-gray-800 active--exact active"><span class="font-bold hidden sm:inline-block sm:visible">Site Home</span></a><nav class="nav flex flex-col sm:flex-row"><a href="/works" class="nav__link uppercase text-black text-sm font-medium p-2 hover:text-red-600">Works</a><a href="/about" class="nav__link uppercase text-black text-sm font-medium p-2 hover:text-red-600">About</a><a href="/blog" class="nav__link uppercase text-black text-sm font-medium p-2 hover:text-red-600">Blog</a><a href="/wp" class="nav__link uppercase text-black text-sm font-medium p-2 hover:text-red-600">WP</a><a href="/kitchen-sink" class="nav__link uppercase text-black text-sm font-medium p-2 hover:text-red-600">Sink</a><a href="/contact" class="nav__link uppercase text-black text-sm font-medium p-2 hover:text-red-600">Contact</a></nav></header>
  </div>
  <p>Meta nav is on top, except on mobile where the meta nav is on the bottom. <br>Via flex-column-reverse on the parent container.</p>
</section>


<section class="section__9">
  <h1>S9 Center center</h1>
  <div  class="flex justify-center items-center" style="height: 300px">
    <div class="item">first item - some more content</div>
  </div>
  <p>justify-content: center + align-items: center</p>
</section>

<section class="section__9b">
  <h1>S9b Center center (align-items)</h1>
  <div  class="flex justify-center items-center" style="height: 300px">
    <div class="item">first item - some more content</div>
    <div class="item">second item</div>
  </div>
  <p>justify-content: center + align-items: center <br>aligns the items inside a flex container along the cross axis just like justify-content does along the main axis.</p>
</section>
!

CSS 

              
                :root {
  --main-bg-color: rgba(255,0,0,.15);
  --div-bg-color: rgba(0,0,0,1);
}
section {
  height: 100vh;
  border: 1px solid #ddd;
  display: flex;
  flex-direction: column;
  justify-content: space-around;
  > div {
    border: 2px solid var(--div-bg-color);
    // padding: 1rem;
  }
}
.item {
  background: var(--main-bg-color);
  border: 2px solid ;
  padding: .5rem
}

.section__1-c {
  .item {
    flex-grow: 1;
    flex-shrink: 1;
    flex-basis: auto;
  }
}

.section__1-d {
  .item {
    flex-grow: 1;
    flex-shrink: 1;
    flex-basis: 0;
  }
}
.section__6 {
  .item:nth-of-type(1) { order: 3; }
  .item:nth-of-type(2) { order: 4; }
  .item:nth-of-type(3) { order: 1; }
  .item:nth-of-type(4) { order: 5; }
  .item:nth-of-type(5) { order: 2; }
  .item:last-child {
    order: -1;
  }
}
.section__7 {
  > div {
    justify-content: center;
    .item {
      // flex-grow: 1;
    }
  }
}




// codepen
body {
  width: 70vw;
  margin: 0 auto;
  display: flex;
  flex-direction: column;
  // flex-direction: column-reverse;
}

h1, p { 
  text-align: center; 
  color: #777; 
  margin: 1em 0;
}
h1 {
  font-size: 1.6em;
  color: black;
}
!

JS 

              
                // https://www.smashingmagazine.com/2018/09/flexbox-sizing-flexible-box/

// https://yoksel.github.io/flex-cheatsheet/
!
999px

Console