Prepare your transition from HTML to XHTML ...

Source for more :

All XML elements must have a closing tag

An element value.

With XML, it is illegal to omit the closing tag

Empty Closed Element
<anemptyelement />

XML tags are case sensitive XHTML : all tags, attributes should be in lowercase

All XML elements must be properly nested

Attribute values must always be quoted

With XML, white space is preserved .... but not in XHTML

With XML, CR / LF is converted to LF

With XML, a new line is always stored as LF.