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HTML 

              
                <div class="legend">
  <h2>Legend</h2>
  <div><div class="legend__padding"></div>padding-box</div>
  <div><div class="legend__content"></div>content-box</div>
</div>

<h2 class="heading">Flex-basis: 0 and flex item with padding </h2>
<div class="container">
  <div class="item flex-2">
    <h3>flex-grow: 2;</h3>
    <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Facere labore, odit fuga itaque neque in, laboriosam distinctio quos molestiae quibusdam a vero delectus dolorem ab ullam rerum culpa iusto veritatis.</p></div>
  <div class="item flex-3 bg-dark padding">
    <h3>flex-grow: 3;</h3>
    Lorem</div>
<div class="item flex-1"><h3>flex-grow: 1;</h3>Lorem isum sit dolor amet</div>
</div>

<h2 class="heading">Flex-basis: 0 and flex item without padding </h2>
<div class="container">
  <div class="item flex-2">
    <h3>flex-grow: 2;</h3>
    Lorem ipsum dolor sit amet, consectetur adipisicing elit. Facere labore, odit fuga itaque neque in, laboriosam distinctio quos molestiae quibusdam a vero delectus dolorem ab ullam rerum culpa iusto veritatis.</div>
  <div class="item flex-3 bg-dark"><div class="padding"><h3>flex-grow: 3;</h3>Lorem</div></div>
<div class="item flex-1"><h3>flex-grow: 1;</h3>Lorem isum sit dolor amet</div>
</div>

<h2 class="heading">Flex-basis: auto</h2>
<div class="container flex-auto">
  <div class="item flex-2">
    <h3>flex-grow: 2;</h3>
    <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Facere labore, odit fuga itaque neque in, laboriosam distinctio quos molestiae quibusdam a vero delectus dolorem ab ullam rerum culpa iusto veritatis.</p></div>
  <div class="item flex-3 bg-dark">
    <h3>flex-grow: 3;</h3>
    Lorem</div>
<div class="item flex-1"><h3>flex-grow: 1;</h3>Lorem isum sit dolor amet</div>
</div>
!

CSS 

              
                * {
  box-sizing: border-box;
}

body {
  font-family: sans-serif;
}

.legend {
  width: 600px;
  margin: 1rem auto;
  background: rgba(89, 190, 205, 0.2);
}

.legend > div {
  display: flex;
  align-items: center;
}

.legend__padding {
  width: 3rem;
  height: 1.5em;
  display: inline-block;
  margin-right: 1rem;
  background: repeating-linear-gradient(-45deg, rgba(50, 127, 190, 0.6), rgba(50, 127, 190, 0.6) 2px, transparent 2px, transparent 5px);
}

.legend__content {
  width: 3rem;
  height: 1.5em;
  display: inline-block;
  margin-right: 1rem;
  background: rgba(89, 190, 205, 0.5);
}

.heading {
  margin-top: 2rem;
  margin-bottom: 0;
  width: 600px;
  margin-left: auto;
  margin-right: auto;
}

.container {
  display: flex;
  width: 600px;
  margin: 1rem auto;
  background: rgba(89, 190, 205, 0.2);
}

.item {
  flex-basis: 0;
  flex-shrink: 1;
  box-sizing: border-box;
}

.flex-auto > .item {
  flex-basis: auto;
}

.flex-1 {
  flex-grow: 1;
}

.flex-2 {
  flex-grow: 2;
}

.flex-3 {
  flex-grow: 3;
}

.bg-dark {
  position: relative;
  background: rgba(89, 190, 205, 0.5);
  background-clip: content-box;
}

.bg-dark:before {
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background: repeating-linear-gradient(-45deg, rgba(50, 127, 190, 0.6), rgba(50, 127, 190, 0.6) 2px, transparent 2px, transparent 5px);
}

.padding {
  /* 
  Padding increases the hypothetical main size of the flex item which gets subtracted from the remaining free space distributed by the flexbox algorithm. This means flex items will not completely match the flex-grow ration.
  
  To avoid this, define borders and padding on a child of the flex item.
  
  To read more about flexbox algorithm see the psecification: https://www.w3.org/TR/css-flexbox-1/#layout-algorithm
  */
  padding: 0 2rem;
}
!

JS
!
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