XHTML is the clean version of HTML. On a modern Linux computer there is no reason to not use it, however if your files go to other environments, as other web servers and client PC you can face problems that nothing arrives. XHTML is still too new for them. It might therefore be wiser to stick with the old HTML.
XHTML is XML. The Extensible Hypertext Markup Language (XHTML) is as HTML but follows a XML DTD and not the SGML DTD. The SGML DTD HTML 4.01 has been converted to XML. The use of XML DTD instead of SGML DTD simplifies the automatic processing: Therefor more tools exist and XHTML should be preferred over HTML. The differences between XHTML and HTML are small and most users and tools do not care what they receive.
The program tidy can convert HTML in XHTML
Example 11.6. XHTML
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <head> <title>XHTML</title> </head> <body> <p>Hello World!</p> </body> </html>
In the example above the first line is intentionally not <?xml version="1.0"?> this would be correct too and cleaner, but can cause some old webservers and browsers to fail