diff --git a/core/modules/responsive_image/src/ResponsiveImageStyleForm.php b/core/modules/responsive_image/src/ResponsiveImageStyleForm.php
index 3badae1..29f7ff5 100644
--- a/core/modules/responsive_image/src/ResponsiveImageStyleForm.php
+++ b/core/modules/responsive_image/src/ResponsiveImageStyleForm.php
@@ -149,7 +149,8 @@ public function form(array $form, FormStateInterface $form_state) {
         $form['keyed_styles'][$breakpoint_id][$multiplier]['sizes'] = array(
           '#type' => 'textfield',
           '#title' => $this->t('Sizes'),
-          '#default_value' => isset($image_style_mapping['image_mapping']['sizes']) ? $image_style_mapping['image_mapping']['sizes'] : '',
+          '#required' => TRUE,
+          '#default_value' => isset($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(
@@ -159,6 +160,7 @@ public function form(array $form, FormStateInterface $form_state) {
         );
         $form['keyed_styles'][$breakpoint_id][$multiplier]['sizes_image_styles'] = array(
           '#title' => $this->t('Image styles'),
+          '#required' => TRUE,
           '#type' => 'checkboxes',
           '#options' => array_diff_key($image_styles, array('' => '')),
           '#description' => $this->t('Select image styles with widths that range from the smallest amount of space this image will take up in the layout to the largest, bearing in mind that high resolution screens will need images 1.5x to 2x larger.'),