it is a simple progression,
there is very little difference.
re: css a bunch of cool extra attributes
re: html a few extra tags to learn and no trailing slashes are required with 'unclosed' elements - and become much more symantic in your mindset if you weren't already
of course if you were extending the html by using other xml in your documents you can't do that of course. however there is an xhtml version of html if you require it.5