Older browsers may not support XHTML, so you should use the HTML syntax. You will also need to provide style definitions for the new elements in HTML5.
Remy Sharp has written a JavaScript that you can use to make HTML5 elements work in Internet Explorer 8 and below. See www.remysharp.com/2009/01/07/html5-enabling-script.
article, aside, canvas, details, figcaption, figure, footer, header, hgroup, menu, nav, section, summary { display: block; }