group-block-vs-cover-block-background-image.post_content.html

<!-- wp:group {"style":{"background":{"backgroundImage":{"url":"http://localhost:8889/wp-content/uploads/2023/09/image.jpeg","id":112,"source":"file","title":"image"}},"spacing":{"padding":{"right":"var:preset|spacing|30","left":"var:preset|spacing|30"}}},"backgroundColor":"contrast","layout":{"type":"constrained","justifyContent":"right"}} -->
<div class="wp-block-group has-contrast-background-color has-background" style="padding-right:var(--wp--preset--spacing--30);padding-left:var(--wp--preset--spacing--30)"><!-- wp:spacer {"height":"284px"} -->
<div style="height:284px" aria-hidden="true" class="wp-block-spacer"></div>
<!-- /wp:spacer -->

<!-- wp:paragraph {"align":"right","style":{"elements":{"link":{"color":{"text":"var:preset|color|base"}}}},"textColor":"base"} -->
<p class="has-text-align-right has-base-color has-text-color has-link-color">No <code>fetchpriority</code> for<br><code>background-image</code> here<br>yet it is the LCP element.</p>
<!-- /wp:paragraph -->

<!-- wp:spacer {"height":"161px"} -->
<div style="height:161px" aria-hidden="true" class="wp-block-spacer"></div>
<!-- /wp:spacer --></div>
<!-- /wp:group -->

<!-- wp:cover {"url":"http://localhost:8889/wp-content/uploads/2023/09/image.jpeg","id":112,"dimRatio":50,"style":{"spacing":{"padding":{"right":"var:preset|spacing|30","left":"var:preset|spacing|30"}}},"layout":{"type":"constrained"}} -->
<div class="wp-block-cover" style="padding-right:var(--wp--preset--spacing--30);padding-left:var(--wp--preset--spacing--30)"><span aria-hidden="true" class="wp-block-cover__background has-background-dim"></span><img class="wp-block-cover__image-background wp-image-112" alt="" src="http://localhost:8889/wp-content/uploads/2023/09/image.jpeg" data-object-fit="cover"/><div class="wp-block-cover__inner-container"><!-- wp:paragraph {"align":"right","placeholder":"Write title…","fontSize":"medium"} -->
<p class="has-text-align-right has-medium-font-size">There is <code>fetchpriority</code> for <br>the background here, but it's<br>not the LCP element.</p>
<!-- /wp:paragraph --></div></div>
<!-- /wp:cover -->

group-block-vs-cover-block-background-image.rendered.html

<div
  class="entry-content wp-block-post-content has-global-padding is-layout-constrained wp-block-post-content-is-layout-constrained"
>
  <div
    class="wp-block-group has-contrast-background-color has-background has-global-padding is-content-justification-right is-layout-constrained wp-container-7 wp-block-group-is-layout-constrained"
    style="
      padding-right: var(--wp--preset--spacing--30);
      padding-left: var(--wp--preset--spacing--30);
      background-image: url(&#039;http://localhost:8889/wp-content/uploads/2023/09/image.jpeg&#039;);
      background-size: cover;
    "
  >
    <div style="height: 284px" aria-hidden="true" class="wp-block-spacer"></div>

    <p
      class="has-text-align-right has-base-color has-text-color has-link-color wp-elements-89e313c61735b4636f00e770bb675cd3"
    >
      No <code>fetchpriority</code> for<br /><code>background-image</code>
      here<br />yet it is the LCP element.
    </p>

    <div style="height: 161px" aria-hidden="true" class="wp-block-spacer"></div>
  </div>

  <div
    class="wp-block-cover"
    style="
      padding-right: var(--wp--preset--spacing--30);
      padding-left: var(--wp--preset--spacing--30);
    "
  >
    <span
      aria-hidden="true"
      class="wp-block-cover__background has-background-dim"
    ></span
    ><img
      fetchpriority="high"
      decoding="async"
      width="979"
      height="1023"
      class="wp-block-cover__image-background wp-image-112"
      alt=""
      src="http://localhost:8889/wp-content/uploads/2023/09/image.jpeg"
      data-object-fit="cover"
      srcset="
        http://localhost:8889/wp-content/uploads/2023/09/image.jpeg         979w,
        http://localhost:8889/wp-content/uploads/2023/09/image-287x300.jpeg 287w,
        http://localhost:8889/wp-content/uploads/2023/09/image-768x803.jpeg 768w
      "
      sizes="(max-width: 979px) 100vw, 979px"
    />
    <div
      class="wp-block-cover__inner-container has-global-padding is-layout-constrained wp-block-cover-is-layout-constrained"
    >
      <p class="has-text-align-right has-medium-font-size">
        There is <code>fetchpriority</code> for <br />the background here, but
        it&#8217;s<br />not the LCP element.
      </p>
    </div>
  </div>
</div>

Screenshot: