The three sources of HTML are:

1) W3C Html (http://www.w3.org/MarkUp/): On this website, you will find pointers to our specifications for HTML/XHTML, guidelines on how to use HTML/XHTML to the best effect, and pointers to related work at W3C

2) HTMLGoodies.com (http://www.htmlgoodies.com/primers/basics.html): This site has excellent tutorials on HTML to help a person understand and start coding HTML.

3) Webmonkey (http://hotwired.lycos.com/webmonkey/authoring/): A website giving in depth information on HTML written by gurus, starting from information on HTML to XML.

New features of HTML 4.0:
The new version includes the capacity to use Cascading Style Sheets (CSS) for precise formatting. The World Wide Web Consortium (W3C) recommends that developers move formatting information out of HTML text and use CSS whenever possible. Other new features are improvements to tables and forms, making them more flexible and more easily controlled. There are also new printing and multimedia features.

HTML 4.0 makes frames an official part of the specification, approving the <FRAME>, <FRAMESET>, and <IFRAME> elements. Supported by popular browsers since Netscape Navigator 2.0 and Internet Explorer 3.0, the HTML specification never defined them until version 4.0. Three elements are now obsolete, ten elements are deprecated (no longer supported), and more than fifteen new elements have been added.

Relationship between XML and HTML:
XML is similar to the language of today's Web pages, the Hypertext Markup Language. Both XML and HTML contain markup symbols to describe the contents of a page or file. HTML, however, describes the content of a Web page (mainly text and graphic images) only in terms of how it is to be displayed and interacted with. For example, the letter "p" placed within markup tags starts a new paragraph. XML describes the content in terms of what data is being described. For example, the word "phone num" placed within markup tags could indicate that the data that followed was a phone number. This means that an XML file can be processed purely as data by a program or it can be stored with similar data on another computer or, like an HTML file, that it can be displayed. For example, depending on how the application in the receiving computer wanted to handle the phone number, it could be stored, displayed, or dialed.

XML is "extensible" because, unlike HTML, the markup symbols are unlimited and self-defining. It is expected that HTML and XML will be used together in many Web applications. XML markup, for example, may appear within an HTML page.

1) http://www.hansenmedia.com/html4_fe.htm
2) http://searchwebservices.techtarget.com/sDefinition/0,,sid26_gci213404,00.html