diff --git a/core/modules/responsive_image/src/ResponsiveImageStyleForm.php b/core/modules/responsive_image/src/ResponsiveImageStyleForm.php
index 6fc982b3236..0d03835e83e 100644
--- a/core/modules/responsive_image/src/ResponsiveImageStyleForm.php
+++ b/core/modules/responsive_image/src/ResponsiveImageStyleForm.php
@@ -160,12 +160,15 @@ class ResponsiveImageStyleForm extends EntityForm {
         $form['keyed_styles'][$breakpoint_id][$multiplier]['sizes'] = array(
           '#type' => 'textfield',
           '#title' => $this->t('Sizes'),
-          '#default_value' => isset($image_style_mapping['image_mapping']['sizes']) ? $image_style_mapping['image_mapping']['sizes'] : '',
+          '#default_value' => isset($image_style_mapping['image_mapping']['sizes']) ? $image_style_mapping['image_mapping']['sizes'] : '100vw',
           '#description' => $this->t('Enter the value for the sizes attribute: for example "(min-width:700px) 700px, 100vw)".'),
           '#states' => array(
             'visible' => array(
               ':input[name="keyed_styles[' . $breakpoint_id . '][' . $multiplier . '][image_mapping_type]"]' => array('value' => 'sizes'),
             ),
+            'required' => array(
+              ':input[name="keyed_styles[' . $breakpoint_id . '][' . $multiplier . '][image_mapping_type]"]' => array('value' => 'sizes'),
+            ),
           ),
         );
         $form['keyed_styles'][$breakpoint_id][$multiplier]['sizes_image_styles'] = array(
@@ -178,6 +181,9 @@ class ResponsiveImageStyleForm extends EntityForm {
             'visible' => array(
               ':input[name="keyed_styles[' . $breakpoint_id . '][' . $multiplier . '][image_mapping_type]"]' => array('value' => 'sizes'),
             ),
+            'required' => array(
+              ':input[name="keyed_styles[' . $breakpoint_id . '][' . $multiplier . '][image_mapping_type]"]' => array('value' => 'sizes'),
+            ),
           ),
         );
 
@@ -221,6 +227,20 @@ class ResponsiveImageStyleForm extends EntityForm {
         // Remove the image style mappings since the breakpoint ID has changed.
         $form_state->unsetValue('keyed_styles');
       }
+
+      // Check that at least 1 image style has been selected when using sizes.
+      foreach ($form_state->getValue('keyed_styles') as $breakpoint_id => $multipliers) {
+        foreach ($multipliers as $multiplier => $image_style_mapping) {
+          if ($image_style_mapping['image_mapping_type'] === 'sizes') {
+            if (empty($image_style_mapping['sizes'])) {
+              $form_state->setError($form['keyed_styles'][$breakpoint_id][$multiplier]['sizes'], 'Provide a value for the sizes attribute.');
+            }
+            if (empty(array_keys(array_filter($image_style_mapping['sizes_image_styles'])))) {
+              $form_state->setError($form['keyed_styles'][$breakpoint_id][$multiplier]['sizes_image_styles'], 'Select at least one image style.');
+            }
+          }
+        }
+      }
     }
   }