XHTML first became a standard on January 26, 2000, in version 1.0.
HTML and XHTML are two different languages used for web and application design. XHTML was a popular choice from about the year 2000, to 2011. HTML 5 is intended to replace XHTML. XHTML followed the basic rules of XML-based languages. HTML (both prior and since) do not.
The World Wide Web Consortium, or the W3C. They set the standards and specifications of HTML and XHTML, and how it should be used.
HTML and XHTML are both markup languages used to markup content for display on web browsers or other compatible devices. HTML stands for HyperText Markup Language and XHTML stands for eXtensible HyperText Markup Language. XHTML was created to help make HTML more XML like in structure and usage as compared to standard HTML.
According to an IBM document ( http://www.ibm.com/developerworks/xml/library/x-ffox15.html ) XHTML 1.1 is supported by Firefox: "Mozilla Firefox 1.0 supports XHTML 1.0 Strict, Transitional, and Frameset, as well as XHTML 1.1 which is based on modularization of XHTML 1.0 Strict. XHTML 2.0 is still in the working draft stage. Some developers wonder how well XHTML 2.0 will be adopted because it's such a significant change from HTML and XHTML 1.x. I recommend that Web developers targeting Firefox (such as for embedded applications) serve XHTML 1.1. Unfortunately, this may not yet be practical for developers targeting the general Web."
HTML is emerging these days as web 2.0 dhtml and xhtml
Embedded
XHTML and CSS are both languages that are used to make web pages. XHTML is very similar to HTML and serves the same purpose. XHTML & HTML are both used to set the structure of the page - to define images, links, text, headlines, etc. CSS (Cascading Style Sheets) is used to define the style of the page - colors, spacing, etc. If you were to think of a web page as a person, XHTML would be the skeleton, and CSS would be the skin, clothing, make-up, etc.
XHTML stands for Extensible Hypertext Markup Language. It is an authoring or presentation language that consists of a set of codes or tags that makes a document able to be displayed on the World Wide Web when read by a Web browser. Besides determining the layout and the structure of the specific Web page, XHTML also defines the links to other pages. It determines where graphics and pictures will be placed and how text will appear and specifies the use of different fonts and italics,
The only disadvantage used to be that browsers would not support it, but since it became the w3c standard most browsers support xhtml meaning there are no actual disadvantages.
The best way to learn how to design a webpage is getting some book and tutorials in XHTML and Photoshop. Go through them,practice coding and start analyzing other web sites.
(1) XHTML 1.0 Strict(2) XHTML 1.0 Transitional(3) XHTML 1.0 Frameset