diff --git a/src/calibre/ebooks/oeb/polish/container.py b/src/calibre/ebooks/oeb/polish/container.py
index 63230b899b..6cd9e47b5b 100644
--- a/src/calibre/ebooks/oeb/polish/container.py
+++ b/src/calibre/ebooks/oeb/polish/container.py
@@ -202,6 +202,9 @@ class Container(object):
         return data
 
     def parse_css(self, data, fname):
+        ''' WARNING: This modifies the CSS tripping out @page rules, comments
+        etc. If you wish to write back the css, you should override the
+        css_preprocessor with a dummy one. '''
         from cssutils import CSSParser, log
         log.setLevel(logging.WARN)
         log.raiseExceptions = False