Common Ideas Between HTML and XHTML

As of this writing, HTML and XHTML are both being used to create Web sites. But there are multiple versions of each, with specific changes and ideas attached. The following table shows the current W3C HTML and XHTML recommendations of note.

HTML & XHTML Versions
Version Introduced Changes from Prior Versions
HTML 4.0 1997 Deprecation of presentational elements in favor of style sheets, awareness of accessibility and alternative device needs, concern with improved rendering of Web documents, introduction of three unique Document Type Definitions (DTDs).
HTML 4.01 1999 Revision of HTML 4.0 fixing minor errors. HTML 4.01 is canonically important because it forms the basis for XHTML 1.0
XHTML 1.0 2000 HTML is now an XML application and authors must begin to write well-formed, valid, and conforming documents
XHTML 1.1 2001 Introduction of modularization and the Ruby Annotation
XHTML 1.0, Second Edition 2002 Not a new version; it brings the XHTML 1.0 Recommendation up to date with comments from the community, ongoing work within the HTML Working Group, and the first edition errata.

HTML 4.0 and 4.01: Concepts

Within HTML 4.0 are specific ideas necessary to study if you're to have a full understanding of the versions and languages that have followed. Specifically, HTML 4.0 provides these critical concepts as follows:

HTML 4.0 really upped the ante in terms of offering real options and alternatives to Web developers interested in also writing documents that conformed to W3C goals. However, many authors have missed learning these important concepts, which lay the foundation for XHTML.

With HTML 4.01, a few errors and editorial changes were introduced. These changes are minor in terms of general concept, but are important because it is upon the updated HTML 4.01 DTDs rather than HTML 4.0 DTDs that XHTML 1.0 is based upon.

XHTML 1.0 and 1.1: Current Ideas and New Directions

XHTML 1.0 is the reformulation of HTML as an XML application. This means that documents as well as syntax must conform strictly to the concepts and DTDs of the language version. The ideas from HTML 4.0, especially the separation of document structure from presentation and issues concerning accessibility and internationalization, are intact in XHTML 1.0. What's more, the three DTD offerings (strict, transitional, and frameset) originally from HTML 4.0 and later refined by HTML 4.01 are essentially the same DTDs found in XHTML 1.0.

XHTML 1.0 is best seen as a transitional language that helps puts professional Web authors in the position of writing specification-oriented markup. It puts browser manufacturers on the hot-seat and tells them "get your acts together!" It also moves us toward the extensible intelligence of XML and away from the limitations of HTML.

But transition also means that readying yourself for XML is very important, too. Fortunately, XHTML contains enough of each to help strengthen your HTML skills and to get those of you unfamiliar with XML more comfortable with its applications. Several of the primary XML concepts introduced in XHTML 1.0 are as follows:

With XHTML 1.1, the concept of separation of structure and presentation is complete. XHTML 1.1 has only one public DTD, based on the Strict DTD found in XHTML 1.0. Web authors also have the option to work with modularization. Modularization breaks HTML down into discrete modules such as text, images, tables, frames, forms, and so forth. The author can choose which modules he or she wants to use and then write a DTD combining those modules into a unique application. This is the first time we really see the extensibility introduced by XML at work, because instead of having only the public DTDs to choose from, authors can now create their own applications.