Index: wp-content/themes/twentythirteen/style.css
===================================================================
--- wp-content/themes/twentythirteen/style.css	(revision 24720)
+++ wp-content/themes/twentythirteen/style.css	(working copy)
@@ -733,6 +733,11 @@
 	font-weight: 300;
 }
 
+.wp-caption .wp-caption-text {
+	width: -webkit-calc(100% - 10px);
+	width:         calc(100% - 10px);
+}
+
 img.wp-smiley,
 .rsswidget img {
 	border: 0;
Index: wp-content/themes/twentytwelve/style.css
===================================================================
--- wp-content/themes/twentytwelve/style.css	(revision 24720)
+++ wp-content/themes/twentytwelve/style.css	(working copy)
@@ -377,6 +377,10 @@
 	line-height: 2;
 	color: #757575;
 }
+.wp-caption .wp-caption-text {
+	width: -webkit-calc(100% - 10px);
+	width:         calc(100% - 10px);
+}
 img.wp-smiley,
 .rsswidget img {
 	border: 0;