How to add custom CSS class to WordPress post featured images

It’s often needed to add custom CSS classes to the thumbnail aka featured image of a post for various reasons. One of the most queried reason is to exclude post featured images from lazy loading and always preloading them.
We can do it with 1 line of PHP code .

<?php 

  add_filter( 'post_thumbnail_html', function($html){
    return preg_replace('/class=([\'"])/','class=$1featured-image ',$html);
  },10,1);
  
  ?>
 

Now we can use featured-image classname wherever we need e.g, we can now add custom styles to all the post featured images, to do that we need to add the CSS rules to the featured-image class.

.featured-image{
  border-radius: 8px;
}

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