HTML and XHTML are very important for us, because now we are using an android phone and everyone use an internet. So the most point is all the website are developed through HTML & XHTML.
HTML is used only for design purpose, in webpage we are seen part in designing by HTML, through we can make a good and good design as per developer mind. so now we are in most using HTML.
XHTML is the advance part of HTML, in this we can do everything and we can make our own tag, they tag are using in developing purpose, the developer define tags, they tags are easy to understand while having problem.
HTML and XHTML is most powerful and advance feature in website.
HTML+ XML is call XHTML
and XML is call Extensible markup Langauge.
HTML= Hypertext Markup Language
XHTML= Extensible hypertext Markup language