body {
  background-image: url('https://placehold.co/600x400/jpg?text=JPEG');
  background-image: image-set(
    url('https://placehold.co/600x400/webp?text=WEBP') type('image/webp'),
    url('https://placehold.co/600x400/jpg?text=JPEG') type('image/jpeg')
  );
}
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