title: Use of rem, em and px on image dimensions
body: <p>I'm looking for some explanation regarding these units' use on image dimensions, but can't find any. I've read in some places to forget about pixels and use only rem/em, but I don't know if it's also applied for images dimensions (width/height).</p>
  
  <p>For example, which one should I use in this case below?</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
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  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;img src="https://i.imgur.com/NReN4xE.png" style="width: 5rem;" /&gt;5rem&lt;br&gt;
  &lt;img src="https://i.imgur.com/NReN4xE.png" style="width: 5em;" /&gt;5em&lt;br&gt;
  &lt;img src="https://i.imgur.com/NReN4xE.png" style="width: 100px;" /&gt;100px&lt;br&gt;</code></pre>
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