diff --git a/docs/requirements.txt b/docs/requirements.txt
index 41b651f..7a60365 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -1,3 +1,4 @@
 jupyter==1.0.0
 nbconvert==7.0.0
+lxml[html_clean]
 octave_kernel