diff --git a/requirements.txt b/requirements.txt
index 6f91090..bc3178f 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -6,6 +6,7 @@ cffi==1.15.0
 chardet==3.0.4
 click==8.0.3
 cryptography==3.3.2
+cssutils==2.4.0
 defusedxml==0.7.1
 Flask==1.1.1
 Flask-Session==0.4.0