A change I missed - In XHTML, 'empty' elements (ones which don't have a closing tag) need to be closed, using a forward slash before the ending >.

XHTML is the XML based successor to HTML. Another syntax issue to note with it is that you have to close paired tags like the p tag, whereas you could get away with not doing this sometimes in HTML.

Links:
Learning XHTML
W3schools XHTML
W3schools XML
Learn XML in 11.5 minutes