What is XHTML?
XHTML is the next generation of HTML. Move up to the new way of creating a web page and web site.
XHTML - eXtensible HyperText Markup Language.
The xmlns attribute is required in XHTML but not in HTML. However, the validator on w3.org does not complain when this attribute is missing from the <html> tag in an XHTML document. That is because "xmlns=http://www.w3.org/1999/xhtml" is a fixed value and will be added to the <html> tag even if you do not include it.
XHTML is less flexible than HTML. Most of the web browsers will present a webpage coded in HTML even if there may be some small dissimilarity in the code. But XHTML is stricter than html in code format.