diff -u b/core/modules/image/image.module b/core/modules/image/image.module
--- b/core/modules/image/image.module
+++ b/core/modules/image/image.module
@@ -287,7 +287,7 @@
     '#height' => $dimensions['height'],
     '#attributes' => $variables['attributes'],
     '#uri' => $style->buildUrl($variables['uri']),
-    '#style_name' => drupal_html_class($variables['style_name']),
+    '#style_name' => $variables['style_name'],
   );
 
   if (isset($variables['alt']) || array_key_exists('alt', $variables)) {
diff -u b/core/modules/system/templates/image.html.twig b/core/modules/system/templates/image.html.twig
--- b/core/modules/system/templates/image.html.twig
+++ b/core/modules/system/templates/image.html.twig
@@ -14,7 +14,7 @@
 #}
 {%
 set classes = [
-  style_name ? 'image-style-' ~ style_name,
+  style_name ? 'image-style-' ~ style_name|clean_class,
 ]
 %}
 <img{{ attributes.addClass(classes) }} />