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@martynchamberlin /index.html Secret
Last active Jan 29, 2018

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How Flexbox Handles Uneven Amounts of Space in justify-content: center
<h2>The Question</h2>
<p>In flexbox, if <code>align-items: center</code> is declared, and a child has a width that cannot be perfectly centered, where does the extra pixel go? <em>Note: on a retina device, the browser can split the "pixels" so this is a non-issue there</em>.</p>
<h2>The Answer</h2>
<h3>In Horizontal spacing</h3>
<p>The left gets the extra pixel.</p>
<div class="flex horizontal outer">
<div class="inner"></div>
<h3>In Horizontal spacing done with a different implementation</h3>
<p>The left gets the extra pixel.</p>
<div class="flex horizontal-alt outer">
<div class="inner"></div>
<h3>In Vertical spacing</h3>
<p>The top gets the extra pixel.</p>
<div class="flex vertical outer">
<div class="inner"></div>
<p>What about a simple <code>margin: 0 auto</code> situation? In that case, the left gets the extra pixel as well.</p>
<div class="horizontal outer zero-auto">
<div class="inner">
* {
font-size: 17px;
h3, p {
margin: 0 0 10px;
.outer {
height: 10px;
width: 10px;
background: red;
margin-bottom: 10px;
.flex {
display: flex;
justify-content: center;
.inner {
background: white;
.horizontal-alt {
flex-direction: column;
align-items: center;
.horizontal {
flex-direction: row;
.horizontal .inner, .horizontal-alt .inner {
width: 3px;
height: 100%;
.vertical {
flex-direction: column;
.vertical .inner {
width: 100%;
height: 3px;
.inner.second {
background: green;
.vertical .second {
height: 2px;
.horizontal .second {
width: 2px;
.zero-auto .inner {
margin: 0 auto;
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