diff --git a/README.md b/README.md
index 34bd170..1ac44c5 100644
--- a/README.md
+++ b/README.md
@@ -13,7 +13,10 @@ This module requires the the following PHP extension:
 ## Installation
  1. Install the module.
  2. Copy the contents of the .htaccess file in the module root to your 
- site-wide .htaccess.
+ site-wide .htaccess.*
+
+ \* Images displayed using Drupal core's "Responsive image" formatter
+ don't depend on the .htaccess mechanism to serve WebP derivatives.
 
 ## Maintainers
 * Bart Vanhoutte (Bart Vanhoutte) - https://www.drupal.org/user/1133754
diff --git a/src/Controller/ImageStyleDownloadController.php b/src/Controller/ImageStyleDownloadController.php
index 2ec3cfb..995c33d 100644
--- a/src/Controller/ImageStyleDownloadController.php
+++ b/src/Controller/ImageStyleDownloadController.php
@@ -101,6 +101,39 @@ class ImageStyleDownloadController extends FileDownloadController {
     $target = $request->query->get('file');
     $image_uri = $scheme . '://' . $target;
 
+    // Don't try to generate file if source is missing.
+    if (!file_exists($image_uri)) {
+
+      $path_info = pathinfo($image_uri);
+
+      $possible_image_uris = [
+        // If the image style converted the extension, it has been added to the
+        // original file, resulting in filenames like image.png.jpeg. So to find
+        // the actual source image, we remove the extension and check if that
+        // image exists.
+        $path_info['dirname'] . DIRECTORY_SEPARATOR . $path_info['filename'],
+
+        // Try out the different possible sources for a webp image.
+        str_replace('.webp', '.jpg', $image_uri ),
+        str_replace('.webp', '.jpeg', $image_uri ),
+        str_replace('.webp', '.png', $image_uri )
+      ];
+
+      $source_image_found = FALSE;
+      foreach ($possible_image_uris as $possible_image_uri) {
+        if (file_exists($possible_image_uri)) {
+          $image_uri = $possible_image_uri;
+          $source_image_found = TRUE;
+          break;
+        }
+      }
+
+      if (!$source_image_found) {
+        $this->logger->notice('Source image at %source_image_path not found while trying to generate derivative image at %derivative_path.', ['%source_image_path' => $image_uri, '%derivative_path' => $derivative_uri]);
+        return new Response($this->t('Error generating image, missing source file.'), 404);
+      }
+    }
+
     // Check that the style is defined, the scheme is valid, and the image
     // derivative token is valid. Sites which require image derivatives to be
     // generated without a token can set the
@@ -132,24 +165,6 @@ class ImageStyleDownloadController extends FileDownloadController {
       }
     }
 
-    // Don't try to generate file if source is missing.
-    if (!file_exists($image_uri)) {
-      // If the image style converted the extension, it has been added to the
-      // original file, resulting in filenames like image.png.jpeg. So to find
-      // the actual source image, we remove the extension and check if that
-      // image exists.
-      $path_info = pathinfo($image_uri);
-      $converted_image_uri = $path_info['dirname'] . DIRECTORY_SEPARATOR . $path_info['filename'];
-      if (!file_exists($converted_image_uri)) {
-        $this->logger->notice('Source image at %source_image_path not found while trying to generate derivative image at %derivative_path.', ['%source_image_path' => $image_uri, '%derivative_path' => $derivative_uri]);
-        return new Response($this->t('Error generating image, missing source file.'), 404);
-      }
-      else {
-        // The converted file does exist, use it as the source.
-        $image_uri = $converted_image_uri;
-      }
-    }
-
     // Don't start generating the image if the derivative already exists or if
     // generation is in progress in another thread.
     if (!file_exists($derivative_uri)) {
diff --git a/webp.module b/webp.module
index fc09081..5f6d9c3 100644
--- a/webp.module
+++ b/webp.module
@@ -22,3 +22,30 @@ function webp_help($route_name, RouteMatchInterface $route_match) {
     default:
   }
 }
+
+/**
+ * Implements template_preprocess_responsive_image().
+ */
+function webp_preprocess_responsive_image(&$variables) {
+  $webp_sources = [];
+  foreach ($variables['sources'] as $source) {
+    /** @var \Drupal\Core\Template\Attribute $source */
+    $original_url = $source->offsetGet('srcset')->value();
+    $webp_url = str_ireplace(['.png', '.jpg', '.jpeg'], '.webp', $original_url);
+
+
+
+    // Create a new source pointing to the webp URL.
+    $webp_source = clone $source;
+    $webp_source->offsetSet('srcset', $webp_url);
+    $webp_source->offsetSet('type', 'image/webp');
+    $webp_sources[] = $webp_source;
+  }
+
+  // Add the new sources at the top of the list.
+  $variables['sources'] = array_merge($webp_sources, $variables['sources']);
+
+  // Never output a single image tag, because
+  // we will always have at least two sources.
+  $variables['output_image_tag'] = FALSE;
+}
\ No newline at end of file