diff --git a/web/site-shell.css b/web/site-shell.css
index 1da5a0a..e6fbcec 100644
--- a/web/site-shell.css
+++ b/web/site-shell.css
@@ -82,3 +82,6 @@ footer a:hover{color:var(--laranja2)}
 /* The footer sits at the bottom of the screen even when the page is short. */
 body{display:flex;flex-direction:column;min-height:100vh}
 body>footer{margin-top:auto}
+/* In a column, auto side margins shrink a box to its content: the page's
+   centred blocks keep their full width, up to their max-width, as before. */
+body>.w{width:100%}