Scroll down to the SGML link in the Related Resources section (or read the introductory chapters in this book). The Document Type Definitions (DTDs) that define how HTML functions are written using a Backus-Naur Format (BNF) grammar notation. For more information, please visit
These original DTDs were designed to keep HTML within the structure and limitations of SGML. But recently, HTML's DTDs have been modified to allow non-SGML functionality. For many standards-obedient programmers and designers, this is a violation of proper programming and design. For those simply interested in expanding HTML's capabilities, the modification is a breath of fresh air (and functionality).
A second issue that arises out of proprietary non-SGML-legal HTML extensions is browser envy. Since Netscape introduced its proprietary extensions in 1994, other browser vendors such as Microsoft and NCSA have introduced gimmicks and flashy additions to their own browsers. All this effort has been expended in hopes that if a browser is the only one able to correctly display a Web document which happens to use those proprietary extensions the consumer base will flock to that browser. Although this conflict has spawned some exciting and eye-catching HTML extensions, it has caused more harm than good. Incompatible HTML dialects, stifled competition, and narrow-minded browser development is just some of the fallout from this battle.
Web-based style sheets offer a promising solution to this situation. Style sheets allow the proper separation of a document's structure and content from its form and appearance. With the implementation of style sheets, HTML can return to handling document structure and content (and get back to being proper SGML), and style sheets can handle a document's form and appearance.
This separation allows both authors and readers to influence the presentation of documents without sacrificing platform and device independence. Hopefully, it will also lessen the need to add new HTML tags, both proprietary and standard, to provide ever more sophisticated layout and appearance controls. Every possible layout or design element of a document can be uniquely defined by an attached that is, linked style sheet. This should reduce the pressure to add tags and controls directly to HTML, when such capability will be available through another avenue.