Use CSS in favor of <br> for display math (#6372)

Some CSS to ensure that display math is
displayed centered and on a new line is now included
in the default HTML-based templates; this may be
overridden if the user wants a different behavior.

test/s5-basic.html

@@ -17,6 +17,7 @@
     div.column{display: inline-block; vertical-align: top; width: 50%;}
     div.hanging-indent{margin-left: 1.5em; text-indent: -1.5em;}
     ul.task-list{list-style: none;}
+    .display.math{display: block; text-align: center; margin: 0.5rem auto;}
   </style>
   <!-- configuration parameters -->
   <meta name="defaultView" content="slideshow" />