Why isn't my flex item aligning in the center?
<p>I am trying to center a red box in the middle of the page.</p>

<p>I have set the flex container to 100% in height, and have also set the html,body to 100%, and it still does not align center. </p>

<p>Can anyone please help me understand why its not working? Thanks!</p>

<p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
<div class="snippet-code">
<pre class="snippet-code-css lang-css prettyprint-override"><code>html, body {
  height: 100%;
}
.flex-container {
  display: flex;
  flex-flow: column;
  justify-content: center;
  height: 100%;
}
.box {
  flex: 0 0 100px;
  background: red;
  width: 100px;
}</code></pre>
<pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div class='flex-container'&gt;
    &lt;div class='box'&gt;&lt;/div&gt;
&lt;/div&gt;</code></pre>
</div>
</div>
</p>