diff -u b/core/modules/update/src/ProjectCoreCompatibility.php b/core/modules/update/src/ProjectCoreCompatibility.php
--- b/core/modules/update/src/ProjectCoreCompatibility.php
+++ b/core/modules/update/src/ProjectCoreCompatibility.php
@@ -139,33 +139,62 @@
     foreach ($releases_to_set as &$release) {
       if (!empty($release['core_compatibility'])) {
         $release['core_compatible'] = $this->isCoreCompatible($release['core_compatibility']);
-        $release['core_compatibility_message'] = $this->createMessageFromCoreCompatibility($release['core_compatibility']);
-        $release['core_compatibility_details'] = [
-          '#type' => 'details',
-          '#title' => $release['core_compatible'] ? $this->t('Compatible') : $this->t('Not compatible'),
-          '#open' => FALSE,
-          'message' => [
-            '#markup' => $release['core_compatibility_message'],
-          ],
-          '#attributes' => [
-            'class' => [
-              $release['core_compatible'] ? 'compatible' : 'not-compatible',
-            ],
-          ],
-        ];
-        if (!$release['core_compatible']) {
-          $release['core_compatibility_details']['not-compatible'] = [
-            '#weight' => -10,
-            '#markup' => $this->t('Your version of Drupal core (%version) is not compatible with this update.', ['%version' => $this->existingCoreVersion]),
-            '#prefix' => '<p>',
-            '#suffix' => '</p>',
-          ];
-        }
+        $release['core_compatibility_details'] = $this->createCoreCompatibilityDetails($release['core_compatibility'], $release['core_compatible']);
       }
     }
   }
 
   /**
+   * Creates core a compatibility details element.
+   *
+   * @param string $core_compatibility_constraint
+   *   A semantic version constraint.
+   * @param boolean $core_compatible
+   *   Is the release compatible with the currently installed version of core?
+   *
+   * @return array
+   *   Render array of the core compatibility details element.
+   *
+   * @todo Should this be a twig template?
+   */
+  protected function createCoreCompatibilityDetails($core_compatibility_constraint, $core_compatible) {
+    $element = [
+      '#type' => 'details',
+      'compatibility_range' => [
+        '#markup' => $this->createMessageFromCoreCompatibility($core_compatibility_constraint),
+      ],
+    ];
+    if ($core_compatible) {
+      $element += [
+        '#title' => $this->t('Compatible'),
+        '#open' => FALSE,
+      ];
+      $compatibility_intro = $this->t('Your version of Drupal core (%version) is compatible with this update.', ['%version' => $this->existingCoreVersion]);
+    }
+    else {
+      $element += [
+        '#title' => $this->t('Not compatible'),
+        '#open' => TRUE,
+      ];
+      $compatibility_intro = $this->t('Your version of Drupal core (%version) is not compatible with this update.', ['%version' => $this->existingCoreVersion]);
+    }
+    $element += [
+      'intro' => [
+        '#weight' => -10,
+        '#markup' => $compatibility_intro,
+        '#prefix' => '<p>',
+        '#suffix' => '</p>',
+      ],
+      '#attributes' => [
+        'class' => [
+          $core_compatible ? 'compatible' : 'not-compatible',
+        ],
+      ],
+    ];
+    return $element;
+  }
+
+  /**
    * Creates core a compatibility message from a semantic version constraint.
    *
    * @param string $core_compatibility_constraint