diff --git a/book/source/_static/epub/css/custom.css b/book/source/_static/epub/css/custom.css
index 40ca147..bd7da7c 100644
--- a/book/source/_static/epub/css/custom.css
+++ b/book/source/_static/epub/css/custom.css
@@ -3,6 +3,9 @@ SPDX-FileCopyrightText: 2023 The "Notes on OpenPGP" project
 SPDX-License-Identifier: CC0-1.0
 */
 
+/*
+Admonitions
+*/
 div.admonition {
   background-color: lightgray;
 }