Align both vertically and horizontally using Bootstrap flex : <p>After looking through the bootstrap 4 Flex doc, I can't seem to get the "align-item-center" property to work. I tested pretty much every example available online but really can't seem to change the vertical alignment of anything, it's driving me nuts. I join a simple HTML example I'm trying to make work. </p>

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<pre class="snippet-code-css lang-css prettyprint-override"><code>.d-flex {
  display: -ms-flexbox !important;
  display: flex !important;
}

.justify-content-center {
  -ms-flex-pack: center !important;
  justify-content: center !important;
}

.align-items-center {
  -ms-flex-align: center !important;
  align-items: center !important;
}</code></pre>
<pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div class="d-flex justify-content-center align-items-center"&gt;
  &lt;p&gt; This should be centered right ?&lt;/p&gt;
&lt;/div&gt;</code></pre>
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<p>I'm using Bootstrap 4.4.1 and also have installed popper.js@latest and jquery@latest just to be sure. For further precision I'm only using a simple textline now but in the future I'll add elements whose size are not known until runtime.</p>
