make it look nicer

Author: rachelandrew

transitions/js-transitions.html

@@ -18,13 +18,13 @@
         }
 
         main {
+            box-sizing: border-box;
             display: flex;
             justify-content: center;
             align-items: center;
-            max-width: 960px;
+            max-width: 660px;
             height: 400px;
             border: 1px solid #ccc;
-            margin: 0 auto;
             padding: 20px;
         }