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HTML

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Insecure Resource

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CSS

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CSS Base

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JavaScript

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Babel includes JSX processing.

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HTML 

              
                <div class="intro">
  <p class="mw">This is a run of text in Merriweather.</p>
  <p class="gg">This is a run of text in Georgia.</p>
  <p class="gg-adj">This is a run of text in Georgia with font-size-adjust set to 0.555, which is the (estimated) aspect ratio for Merriweather.</p>
</div>

<div class="fallback">
  <p>This is a run of text that's supposed to be set in <em>Noticia Text</em> but the font doesn't exist, so the fallback of <em>Georgia</em> has been invoked, with the font-size-adjust set to 0.530, which is the estimated aspect ratio of Noticia Text. Georgia has an estimated aspect ratio of 0.481.
</div>
!

CSS 

              
                @import url('https://fonts.googleapis.com/css?family=Merriweather');

body {
  padding: 1em;
}

@supports not (font-size-adjust: 0.5) {
  body::before {
    content: 'If you see this message, it means the demo is not working as the browser you are using does not support font-size-adjust.';
    display: block;
    background-color: coral;
    padding: 0.5em;
    margin-bottom: 1em;
  }
}

div {
  padding: 1em;
  line-height: 1.5;
}

.intro {
  border: 0.25em dotted;
  
  p:not(:last-of-type) {
    margin-bottom: 1em;
  }
}

.mw {
  font-family: Merriweather;
}

.gg {
  font-family: Georgia;
}

.gg-adj {
  font-family: Georgia;
  font-size-adjust: 0.555;
}

.fallback {
  font-family: 'Noticia Text', Georgia, serif;
  font-size-adjust: 0.530;
}
!

JS 

              
                /* Had a little chat with Robin Rendle earlier today about the `font-size-adjust` property and decided to make a demo to demonstrate it. MDN actually has a pretty good demo already at: https://developer.mozilla.org/en-US/docs/Web/CSS/font-size-adjust

So every font has an aspect ratio, which is the ratio of the height of the lower-case x against the height of the font in px. `font-size-adjust` lets us set the aspect ratio of the fall-back font to that of the first choice font.

Resource links:
https://www.cs.tut.fi/~jkorpela/x-height.html
*/
!
999px

Console