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:

Deprecation of presentational elements in favor of style sheets. Consider this the heart and soul of contemporary Web design. HTML 4.0 clearly states that the separation of structure and presentation are an imperative goal in order for Web authoring to progress. CSS is the suggested alternative, which in HTML 4.0's emergence in 1997 was more problematic due to browser support than it is today.
Awareness of Accessibility and Internationalization. HTML 4.0 is very concerned with ensuring that pages are available to those individuals using alternative user agents, whether due to a disability or simply by virtue of circumstance. Internationalization concerns are brought forth in HTML 4.0, and discussions about how to Internationalize and Globalize HTML and XHTML are ongoing.
Improved rendering of Web documents. HTML 4.0 added several elements, specifically in terms of tables, that aid in accelerated interpretation and rendering of markup.
Introduction of three unique DTDs. With HTML 4.0 came the concept of three unique public DTDS: Strict, Transitional, and Frameset. The Strict DTD is HTML 4.0 at its most ideal, with the presentation of a document relying on CSS almost entirely. The Transitional DTD allows for the use of deprecated elements, understanding the transitional need for presentation in HTML. Finally, the Frameset DTD formalizes the use of frames in HTML 4.0 and provides a specific set of rules for their implementation.

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:

Reintroducing structure back into the language. Picking up on the SGML and XML idea that documents should be written in conformance with the rules set out within the languages, XHTML insists upon a variety of syntax and semantic rules that must be adhered to. One such rule is the idea of well-formed.
Providing authors with incentives to validate documents. The validation of documents is somewhat controversial for a number of reasons. Certain people believe that validation is an unnecessary part of the process so long as the documents are well-formed. However, I personally feel that validation is a powerful learning tool that helps us find our mistakes, fix them, and in the process, understand the way a specific DTD works. Validation, therefore, is encouraged throughout.
Accommodating New Devices. Part of the drive to accommodate XML in the Web development environment has to do with an intriguing phenomenon. If the 90s were the years of information explosion and the movement of the PC from the workplace to the home, this decade will be known for the movement away from the desktop.

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.