<p class="box1">
  Lorem ipsum dolor sit amet consectetur adipisicing elit. Voluptates neque iure atque perferendis harum, excepturi, amet laboriosam nam quidem et eligendi reprehenderit similique eius maiores animi voluptatem error ab in.
</p>

<p class="box2">
  Lorem ipsum dolor sit amet consectetur adipisicing elit. Voluptates neque iure atque perferendis harum, excepturi, amet laboriosam nam quidem et eligendi reprehenderit similique eius maiores animi voluptatem error ab in.
</p>

.box1 {
  color: blue;
  /* Supported browser will show black color here: */
  color: var(--my-color); 
}

.box2 {
  color: blue;  
  /* Supported browser will show blue color here: */
  color: var(--my-color, blue);
}

External CSS

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External JavaScript

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