diff --git a/style/main.css b/style/main.css
index 8e2babe..2814b05 100644
--- a/style/main.css
+++ b/style/main.css
@@ -193,6 +193,10 @@ main.not-found {
   color: var(--ink-dim);
   font-size: 1.05rem;
   max-width: 46ch;
+  /* The measure cap shrinks the box below the copy column; without
+     auto margins the box left-anchors and its centered text centers
+     in the wrong frame. */
+  margin-inline: auto;
 }
 
 /* A content-shipped hero module (site.yaml hero.kind: module) draws