diff --git a/src/calibre/srv/fast_css_transform.cpp b/src/calibre/srv/fast_css_transform.cpp
index 3fae1a81b3..4b6afb0a80 100644
--- a/src/calibre/srv/fast_css_transform.cpp
+++ b/src/calibre/srv/fast_css_transform.cpp
@@ -5,6 +5,8 @@
  * Distributed under terms of the GPL3 license.
  */
 
+// See https://www.w3.org/TR/css-syntax-3
+
 #define PY_SSIZE_T_CLEAN
 #define UNICODE
 #define _UNICODE