XHTML was developed by W3C HTML Working Group with Steven Pemberton as the group chair.

There are 77 tags defined in XHTML 1.0:

a abbr acronym address area b base bdo big blockquote body br button caption cite code col colgroup dd del dfn div dl dt em fieldset form h1 h2 h3 h4 h5 h6 head hr html i img input ins kbd label legend li link map meta noscript object ol optgroup option p param pre q samp script select small span strong style sub sup table tbody td textarea tfoot th thead title tr tt ul var.

CSS (Cascading Style Sheets) is a technical specification that allows XHTML document authors to attach formatting style sheets to XHTML documents. When XHTML documents are viewed as Web pages through Web browsers, the attached style sheets will alter the default style sheets embedded in browsers.

SGML (Standard Generalized Markup Language) is a metalanguage in which one can define markup languages, like HTML, for documents. SGML is a descendant of IBM’s Generalized Markup Language (GML), developed in the 1960s by Charles Goldfarb, Edward Mosher and Raymond Lorie (whose surname initials also happen to be GML). XHTML is not directly related to SGML. But it is indirectly to SGML through HTML.

HTTP (Hyper­Text Transfer Protocol) is an Internet communication protocol, that allows a viewer’s computer talking to a publisher’s computer to fetch XHTML documents. The viewer’s computer issues a HTTP request message to the publisher computer first. Then the publisher returns a HTTP response message back to the viewer’s computer with the requested XHTML document included in the message.

The Web is a publisher technology based on the Internet to allow publishers to publish hyper linked documents to be viewed by Internet users. XHTML is the next generation language that used to compose those hyper linked documents. In another word, XHTML is the technical language that controls Web page contents and hyper links.

Serving XHTML with a MIME type of text/html is wrong.

The whole point of XHTML is that it’s XML so that you can benefit from namespaces and the like.

In particular, ‘text/html’ is NOT suitable for XHTML Family document types that adds elements and attributes from foreign namespaces, such as XHTML+MathML [XHTML+MathML].

Two choices:

1. XHTML 1.0 served as application/xhtml+xml to conforming UAs, and text/html to Internet Explorer

2. HTML 4.01, served as text/html 

XHTML 1.1 is not an option because it mandates a MIME type of application/xhtml+xml which is incompatible with Internet Explorer

XHTML is not very different from HTML 4.01, so bringing your code up to the 4.01 standard is a good start. In addition, you should start NOW to write your HTML code in lowercase letters. 

The Most Important Differences:

The basic tool you need to write XHTML documents is any text editor, like notepad on Windows system, or vi on Unix system.If you are looking for some advanced tools to help you writing XHTML documents, you can select any one from the following three categories of XHTML editors:

XHTML editors can also be loaded with additional functionalities to allow you work with other Web technologies like CSS and JavaScript, or publish the XHTML documents to your Web server.

An application may wish to support only a subset of XHTML. For example a mobile phone, an Internet TV or even a Web-aware cooker may only require a subset of XHTML. Also modularity makes it easier to deploy new developments.

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