The "important" boxes on the web pages all hang over to the right compared to all the other boxes.  This is a result of the red highlight on the left side.

My proposed fixes is add a calculated width in a later element.  I did this to fail more gracefully in browsers that don't support calc.  Note the spaces in "90% - 11px" are required. 

--- docbook.css.orig	2019-07-27 15:56:36.000000000 +1000
+++ docbook.css	2019-07-27 16:07:41.000000000 +1000
@@ -334,14 +334,18 @@
 .important {
 	font-style:italic;
 	border: 1px solid #a00;
 	border-left: 12px solid #c00;
 	width: 90%;
 }
 
+.important {
+	width: calc(90% - 11px);
+}
+
 .warning {
 	color: #9F1313;