HTML is HyperText Markup Language and XHTML is eXtend HyperText Markup Language. From the name we can know XHTML is newer than HTML and have something added.
HTML 3.2, HTML 4.0 and HTML 4.0.1 is Original Markup language that have flexible syntax but difficult to parse by program. Why? because HTML tag syntax is
HTML TAG USAGE:
<{command} [attribute[="..."] [attribute[="..."] [...]]]>[...</{command}>]
Are you see what make it difficult? ... Yes it is tag and attribute like e.g. <img> <br> or <hr>.
HTML TAG USAGE:
<{command} [attribute[="..."] [attribute[="..."] [...]]]>[...</{command}>]
*** problem part is hilighting
HTML tag below start without close tag because it not need to have. But it make browser or another program hard to parse to use (Microsoft try to automatic fill it before display). but in XHTML, it based on XML. What it's mean? It mean HTML tag must have end tag or use single tag style. It syntax be
XHTML TAG USAGE:
(<{command} [attribute="" [attribute=""[...]]]>...|<{command} [attribute="" [attribute="" [...]]]/>)
It make you must close all your open tag else use single tag. However if you not use it in strictly mode you also use it like HTML.
Not only "Tag" that make it difference. If I have more time, I'll talk to that topic again
