diff --git a/core/modules/responsive_image/responsive_image.module b/core/modules/responsive_image/responsive_image.module
index ea99ca1..57d8fec 100644
--- a/core/modules/responsive_image/responsive_image.module
+++ b/core/modules/responsive_image/responsive_image.module
@@ -376,7 +376,7 @@ function responsive_image_build_source_attributes(ImageInterface $image, array $
           if (is_null($dimensions['width'])) {
             throw new \LogicException(String::format('Could not determine image width for @file using image style with ID: @image_style_name. This image style can not be used for a responsive image style mapping using the \'sizes\' attribute.', array('@file' => $image->getSource(), '@image_style_name' => $image_style_name)));
           }
-          $srcset[(string) $dimensions['width']] = file_create_url(_responsive_image_image_style_url($image_style_name, $image->getSource())) . ' ' . $dimensions['width'] . 'w';
+          $srcset[intval($dimensions['width'])] = file_create_url(_responsive_image_image_style_url($image_style_name, $image->getSource())) . ' ' . $dimensions['width'] . 'w';
           $sizes = array_merge(explode(',', $image_style_mapping['image_mapping']['sizes']), $sizes);
         }
         break;
@@ -386,7 +386,7 @@ function responsive_image_build_source_attributes(ImageInterface $image, array $
         $derivative_mime_type = responsive_image_get_mime_type($image_style_mapping['image_mapping'], $extension);
         $derivative_mime_types[] = $derivative_mime_type;
         // Add the image source with its multiplier.
-        $srcset[Unicode::substr($multiplier, 0, -1)] = file_create_url(_responsive_image_image_style_url($image_style_mapping['image_mapping'], $image->getSource())) . ' ' . $multiplier;
+        $srcset[intval(Unicode::substr($multiplier, 0, -1) * 100)] = file_create_url(_responsive_image_image_style_url($image_style_mapping['image_mapping'], $image->getSource())) . ' ' . $multiplier;
         break;
     }
   }
diff --git a/core/modules/responsive_image/src/Tests/ResponsiveImageFieldDisplayTest.php b/core/modules/responsive_image/src/Tests/ResponsiveImageFieldDisplayTest.php
index 42f4c47..45a418d 100644
--- a/core/modules/responsive_image/src/Tests/ResponsiveImageFieldDisplayTest.php
+++ b/core/modules/responsive_image/src/Tests/ResponsiveImageFieldDisplayTest.php
@@ -124,14 +124,19 @@ protected function addTestImageStyleMappings($empty_styles = FALSE) {
           'image_mapping_type' => 'image_style',
           'image_mapping' => RESPONSIVE_IMAGE_EMPTY_IMAGE,
         ))
+        // Test the output with a 1.5x multiplier.
+        ->addImageStyleMapping('responsive_image_test_module.mobile', '1.5x', array(
+          'image_mapping_type' => 'image_style',
+          'image_mapping' => 'thumbnail',
+        ))
         // Test the output of the 'sizes' attribute.
         ->addImageStyleMapping('responsive_image_test_module.narrow', '1x', array(
           'image_mapping_type' => 'sizes',
           'image_mapping' => array(
             'sizes' => '(min-width: 700px) 700px, 100vw',
             'sizes_image_styles' => array(
-              'large' => 'large',
-              'medium' => 'medium',
+              'large',
+              'medium',
             ),
           ),
         ))
@@ -159,8 +164,9 @@ protected function doTestResponsiveImageFieldFormatters($scheme, $empty_styles =
     $node_storage = $this->container->get('entity.manager')->getStorage('node');
     $field_name = Unicode::strtolower($this->randomMachineName());
     $this->createImageField($field_name, 'article', array('uri_scheme' => $scheme));
-    // Create a new node with an image attached.
-    $test_image = current($this->drupalGetTestFiles('image'));
+    // Create a new node with an image attached. Make sure we use a large image
+    // so the scale effects of the image styles always have an effect.
+    $test_image = current($this->drupalGetTestFiles('image', 39325));
     $nid = $this->uploadNodeImage($test_image, $field_name, 'article');
     $node_storage->resetCache(array($nid));
     $node = $node_storage->load($nid);
@@ -170,8 +176,8 @@ protected function doTestResponsiveImageFieldFormatters($scheme, $empty_styles =
     $image = array(
       '#theme' => 'image',
       '#uri' => $image_uri,
-      '#width' => 40,
-      '#height' => 20,
+      '#width' => 360,
+      '#height' => 240,
     );
     $default_output = str_replace("\n", NULL, drupal_render($image));
     $this->assertRaw($default_output, 'Default formatter displaying correctly on full node view.');
@@ -190,8 +196,8 @@ protected function doTestResponsiveImageFieldFormatters($scheme, $empty_styles =
     $image = array(
       '#theme' => 'image',
       '#uri' => $image_uri,
-      '#width' => 40,
-      '#height' => 20,
+      '#width' => 360,
+      '#height' => 240,
     );
     $default_output = '<a href="' . file_create_url($image_uri) . '">' . drupal_render($image) . '</a>';
     $this->drupalGet('node/' . $nid);
@@ -237,6 +243,9 @@ protected function doTestResponsiveImageFieldFormatters($scheme, $empty_styles =
       $this->assertRaw('<!--[if IE 9]></video><![endif]-->');
       // Assert the empty image is present.
       $this->assertRaw('data:image/gif;base64,R0lGODlhAQABAIABAP///wAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==');
+      $thumbnail_style = ImageStyle::load('thumbnail');
+      // Assert the output of the 'srcset' attribute (small multipliers first).
+      $this->assertRaw('data:image/gif;base64,R0lGODlhAQABAIABAP///wAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw== 1x, ' . $thumbnail_style->buildUrl($image_uri) . ' 1.5x');
       $this->assertRaw('/styles/medium/');
       // Assert the output of the breakpoints.
       $this->assertRaw('media="(min-width: 0px)"');
@@ -244,6 +253,9 @@ protected function doTestResponsiveImageFieldFormatters($scheme, $empty_styles =
       // Assert the output of the 'sizes' attribute.
       $this->assertRaw('sizes="(min-width: 700px) 700px, 100vw"');
       $this->assertPattern('/media="\(min-width: 560px\)".+?sizes="\(min-width: 700px\) 700px, 100vw"/');
+      // Assert the output of the 'srcset' attribute (small images first).
+      $medium_style = ImageStyle::load('medium');
+      $this->assertRaw($medium_style->buildUrl($image_uri) . ' 220w, ' . $large_style->buildUrl($image_uri) . ' 360w');
       $this->assertRaw('media="(min-width: 851px)"');
     }
     $this->assertRaw('/styles/large/');
@@ -251,6 +263,7 @@ protected function doTestResponsiveImageFieldFormatters($scheme, $empty_styles =
     $this->assertTrue(in_array('config:responsive_image.styles.style_one', $cache_tags));
     if (!$empty_styles) {
       $this->assertTrue(in_array('config:image.style.medium', $cache_tags));
+      $this->assertTrue(in_array('config:image.style.thumbnail', $cache_tags));
       $this->assertRaw('type="image/png"');
     }
     $this->assertTrue(in_array('config:image.style.large', $cache_tags));
diff --git a/core/modules/responsive_image/tests/modules/responsive_image_test_module/responsive_image_test_module.breakpoints.yml b/core/modules/responsive_image/tests/modules/responsive_image_test_module/responsive_image_test_module.breakpoints.yml
index 247ab23..0d32372 100644
--- a/core/modules/responsive_image/tests/modules/responsive_image_test_module/responsive_image_test_module.breakpoints.yml
+++ b/core/modules/responsive_image/tests/modules/responsive_image_test_module/responsive_image_test_module.breakpoints.yml
@@ -4,6 +4,7 @@ responsive_image_test_module.mobile:
   weight: 0
   multipliers:
     - 1x
+    - 1.5x
     - 2x
 responsive_image_test_module.narrow:
   label: narrow
@@ -11,6 +12,7 @@ responsive_image_test_module.narrow:
   weight: 1
   multipliers:
     - 1x
+    - 1.5x
     - 2x
 responsive_image_test_module.wide:
   label: wide
@@ -18,4 +20,5 @@ responsive_image_test_module.wide:
   weight: 2
   multipliers:
     - 1x
+    - 1.5x
     - 2x