diff --git a/h5p-default-storage.class.php b/h5p-default-storage.class.php
index bcd7b1a..27b250b 100644
--- a/h5p-default-storage.class.php
+++ b/h5p-default-storage.class.php
@@ -178,7 +178,7 @@ class H5PDefaultStorage implements \H5PFileStorage {
           $content .= preg_replace_callback(
               '/url\([\'"]?([^"\')]+)[\'"]?\)/i',
               function ($matches) use ($cssRelPath) {
-                  if (preg_match("/^([a-z0-9]+:)?\//i", $matches[1]) === 1) {
+                  if (preg_match("/^(data:|([a-z0-9]+:)?\/)/i", $matches[1]) === 1) {
                     return $matches[0]; // Not relative, skip
                   }
                   return 'url("../' . $cssRelPath . $matches[1] . '")';