<h1>flex-direction: row;</h1>
<section class="container row">
  <div class="item">1</div>
  <div class="item">2</div>
  <div class="item">3</div>
</section>

<h1>flex-direction: row-reverse;</h1>
<section class="container row-reverse">
  <div class="item">1</div>
  <div class="item">2</div>
  <div class="item">3</div>
</section>

<h1>flex-direction: column;</h1>
<section class="container column">
  <div class="item">1</div>
  <div class="item">2</div>
  <div class="item">3</div>
</section>

<h1>flex-direction: column-reverse;</h1>
<section class="container column-reverse">
  <div class="item">1</div>
  <div class="item">2</div>
  <div class="item">3</div>
</section>
.row {
  flex-direction: row;
}
.row-reverse {
  flex-direction: row-reverse;
}
.column {
  flex-direction: column;
}
.column-reverse {
  flex-direction: column-reverse;
}

/* Flex Container */
.container {
  max-width: 400px;
  margin: 0 auto;
  display: flex;
  border: 1px solid #ccc;
}
/* Flex Item */
.item {
  /* O flex: 1; é necessário para que cada item se expanda ocupando o tamanho máximo do container. */
  flex: 1;
  margin: 5px;
  background: tomato;
  text-align: center;
  font-size: 1.5em;
}

h1 {
  text-align: center;
  margin: 20px 0 0 0;
  font-size: 1.25em;
  font-weight: normal;
}

body {
  font-family: monospace;
  color: #333;
}

External CSS

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External JavaScript

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