diff --git a/core/modules/responsive_image/responsive_image.module b/core/modules/responsive_image/responsive_image.module
index 2a0faeb..068f3ea 100644
--- a/core/modules/responsive_image/responsive_image.module
+++ b/core/modules/responsive_image/responsive_image.module
@@ -216,12 +216,12 @@ function template_preprocess_responsive_image(&$variables) {
  * can contain mapping definitions of mixed types (both 'image_style' and
  * 'sizes'). For example:
  * @code
- * $responsiveImgMapping = entity_create('responsive_image_mapping', array(
+ * $responsive_img_mapping = ResponsiveImageMapping::create(array(
  *   'id' => 'mapping_one',
  *   'label' => 'Mapping One',
  *   'breakpoint_group' => 'responsive_image_test_module',
  * ));
- * ->addMappingDefinition('responsive_image_test_module.mobile', '1x', array(
+ * $responsive_img_mapping->addMappingDefinition('responsive_image_test_module.mobile', '1x', array(
  *   'image_mapping_type' => 'image_style',
  *   'image_mapping' => 'thumbnail',
  * ))
@@ -237,12 +237,11 @@ function template_preprocess_responsive_image(&$variables) {
  * ))
  * ->save();
  * @endcode
- * The above responsive image mapping will result in a <picture> tag similar to
- * this:
+ * The above responsive image mapping will result in a <picture> tag like this:
  * @code
  * <picture>
  *   <source media="(min-width: 0px)" srcset="sites/default/files/styles/thumbnail/image.jpeg" />
- *   <source media="(min-width: 560px)" sizes="(min-width: 700px) 700px, 100vw" srcset="sites/default/files/styles/large/image.jpeg 480w, sites/default/files/styles/medium/image.jpeg 220w"/>
+ *   <source media="(min-width: 560px)" sizes="(min-width: 700px) 700px, 100vw" srcset="sites/default/files/styles/large/image.jpeg 480w, sites/default/files/styles/medium/image.jpeg 220w" />
  *   <img srcset="fallback.jpeg" />
  * </picture>
  * @endcode
@@ -298,11 +297,8 @@ function template_preprocess_responsive_image(&$variables) {
  * <source media="(min-width:0px)" srcset="img1.jpeg 50w 1x, img2.jpeg=100w 1x" />
  * @endcode
  *
- * The data structure we use to store the mapping information is a
- * multidimensional array. The first level is the breakpoint, the second level
- * is the multiplier. Since the specs do not allow width descriptors
- * and multipliers combined inside one 'srcset' attribute, we either have to use
- * something like
+ * Note: Since The specs do not allow width descriptors and multipliers combined
+ * inside one 'srcset' attribute, we either have to use something like
  * @code
  * <source [...] srcset="image1.jpeg 1x, image2.webp 2x, image3.jpeg 3x" />
  * @endcode
@@ -316,17 +312,17 @@ function template_preprocess_responsive_image(&$variables) {
  * multiplier) so the array contains an entry for breakpointA.1x and
  * breakpointA.2x. If we would output those we will end up with something like
  * @code
- * <source [...] sizes="(min-width: 40em) 80vw, 100vw" srcset="a1.jpeg 300w 1x, a2.jpeg 600w 1x, a3.jpeg 1200w 1x, b1.jpeg 300w 2x, b2.jpeg 600w 2x, b3.jpeg 1200w 2x" />
+ * <source [...] sizes="(min-width: 40em) 80vw, 100vw" srcset="a1.jpeg 300w 1x, a2.jpeg 600w 1x, a3.jpeg 1200w 1x, b1.jpeg 250w 2x, b2.jpeg 680w 2x, b3.jpeg 1240w 2x" />
  * @endcode
  * which is illegal. So the solution is to merge both arrays into one and
  * disregard the multiplier. Which, in this case, would output
  * @code
- * <source [...] sizes="(min-width: 40em) 80vw, 100vw" srcset="image1.jpeg 300w, image2.webp 600w, image3.jpeg 1200w" />
+ * <source [...] sizes="(min-width: 40em) 80vw, 100vw" srcset="b1.jpeg 250w, a1.jpeg 300w, a2.jpeg 600w, b2.jpeg 680w, a3.jpeg 1200w,  b3.jpeg 1240w" />
  * @endcode
  * See http://www.w3.org/html/wg/drafts/html/master/embedded-content.html#image-candidate-string
  * for further information.
  *
- * @param ImageInterface $image
+ * @param \Drupal\Core\Image\ImageInterface $image
  *   The image to build the <source> tags for.
  * @param array $variables
  *   An array with the following keys:
diff --git a/core/modules/responsive_image/src/Entity/ResponsiveImageMapping.php b/core/modules/responsive_image/src/Entity/ResponsiveImageMapping.php
index beca8d2..bf15443 100644
--- a/core/modules/responsive_image/src/Entity/ResponsiveImageMapping.php
+++ b/core/modules/responsive_image/src/Entity/ResponsiveImageMapping.php
@@ -62,10 +62,11 @@ class ResponsiveImageMapping extends ConfigEntityBase implements ResponsiveImage
    * Each mapping definition array contains the following keys:
    *   - image_mapping_type: Either 'image_style' or 'sizes'.
    *   - image_mapping:
-   *     - If image_mapping_type is 'image_style', the image style ID.
+   *     - If image_mapping_type is 'image_style', the image style ID (a
+   *       string).
    *     - If image_mapping_type is 'sizes', an array with following keys:
-   *       - sizes: If image_mapping_type is 'sizes', the value for the
-   *         'sizes' attribute.
+   *       - sizes: If image_mapping_type is 'sizes', the value for the 'sizes'
+   *         attribute.
    *       - sizes_image_styles: The image styles to use for the 'srcset'
    *         attribute.
    *   - breakpoint_id: The breakpoint ID for this mapping definition.
@@ -98,14 +99,6 @@ public function __construct(array $values, $entity_type_id = 'responsive_image_m
    * {@inheritdoc}
    */
   public function addMappingDefinition($breakpoint_id, $multiplier, array $mapping_definition) {
-    // Default 'image_mapping' to an empty string because the default for
-    // 'image_mapping_type' is 'image_style'. This means 'image_mapping' should
-    // be a string (the image style ID). If 'image_mapping_type' is 'sizes', the
-    // value for 'image_mapping' should be an array.
-    $defaults = array(
-      'image_mapping_type' => 'image_style',
-      'image_mapping' => '',
-    );
     // If there is an existing mapping, overwrite it.
     foreach ($this->mapping_definitions as &$mapping) {
       if ($mapping['breakpoint_id'] === $breakpoint_id && $mapping['multiplier'] === $multiplier) {
diff --git a/core/modules/responsive_image/src/ResponsiveImageMappingForm.php b/core/modules/responsive_image/src/ResponsiveImageMappingForm.php
index cdb5474..c668953 100644
--- a/core/modules/responsive_image/src/ResponsiveImageMappingForm.php
+++ b/core/modules/responsive_image/src/ResponsiveImageMappingForm.php
@@ -108,6 +108,9 @@ public function form(array $form, FormStateInterface $form_state) {
           '#type' => 'container',
         );
         $mapping_definition = $responsive_image_mapping->getMappingDefinition($breakpoint_id, $multiplier);
+        // @todo The image_mapping_type is only temporarily hardcoded, until
+        // support for the other responsive image mapping type ('sizes') is
+        // added in https://www.drupal.org/node/2334387.
         $form['keyed_mappings'][$breakpoint_id][$multiplier]['image_mapping_type'] = array(
           '#type' => 'value',
           '#value' => 'image_style',
diff --git a/core/modules/responsive_image/src/ResponsiveImageMappingInterface.php b/core/modules/responsive_image/src/ResponsiveImageMappingInterface.php
index b35a727..dd6aa54 100644
--- a/core/modules/responsive_image/src/ResponsiveImageMappingInterface.php
+++ b/core/modules/responsive_image/src/ResponsiveImageMappingInterface.php
@@ -100,7 +100,7 @@ public function getMappingDefinition($breakpoint_id, $multiplier);
    * Checks if there is at least one mapping definition defined.
    *
    * @param array $mapping_definition
-   *   The mapping definition array.
+   *   The mapping definition.
    *
    * @return bool
    *   Whether the mapping definition is empty.
@@ -114,8 +114,8 @@ public static function isEmptyMappingDefinition(array $mapping_definition);
    *   The breakpoint ID.
    * @param string $multiplier
    *   The multiplier.
-   * @param string $mapping_definition
-   *   The mapping definition array.
+   * @param array $mapping_definition
+   *   The mapping definition.
    *
    * @return $this
    */