title: font-size difference between chrome and firefox using rem
body: <p>I work on a web project with <code>Bootstrap</code> and I have some trouble with font-size.</p>
  
  <p>Try to run this <code>HTML</code> snippet in chrome and then in firefox with an <strong>mobile view</strong> like iPhone 7:</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-html lang-html prettyprint-override"><code>&lt;!DOCTYPE html&gt;
  &lt;html&gt;
  
  &lt;head&gt;
      &lt;link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css"
          integrity="sha384-ggOyR0iXCbMQv3Xipma34MD+dH/1fQ784/j6cY/iJTQUOhcWr7x9JvoRxT2MZw1T" crossorigin="anonymous"&gt;
  
      &lt;style&gt;
          html {
              font-size: 40px;
              font-size-adjust: none;
          }
  
          h1 {
              font-size: 2rem;
          }
      &lt;/style&gt;
  
  &lt;/head&gt;
  
  &lt;body&gt;
  
      &lt;div&gt;
          &lt;h1&gt;
              test
          &lt;/h1&gt;
      &lt;/div&gt;
  
      &lt;button type="button" class="btn btn-primary"&gt;Send&lt;/button&gt;
  
  &lt;/body&gt;
  
  &lt;/html&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>You will see that in the firefox mobile view the font size is much larger than under chrome. However I initialize well the font size of <code>&lt;html&gt;</code> to a pixel value (here 40px) and then I only use <code>rem</code> has font size values (bootstrap also use rem values for the font size). </p>
  
  <p>Why is the rendering so different? </p>