Html

Browser developers rely upon the HTML standard to program the software that formats and displays common
HTML documents. Authors use the standard to make sure they are writing effective, correct HTML documents.
However, the standard is not always explicit; manufacturers have some leeway in how their browser might display
an element. And to complicate matters, commercial forces have pusheddevelopers to add into their browsers
nonstandard extensions meant to improve the language.
In this book, we explore in detail the syntax, semantics, and idioms of HTML Version 4.01, along with the many
important extensions that are supported in the latest versions of the most popular browsers, so that any aspiring
HTML author can create fabulous documents with a minimum of effort.
1.4 XHTML: WhatIt Is
You've certainly heard of HTML, but did you know that it is one of many other markup languages? Indeed, HTML
is the black sheep in the family of document markup languages. HTML is based on SGML, the Standard
Generalized Markup Language. The powers-that-be created SGML with the intent that it be the one and only
markup metalanguage from which all other document markup elements would becreated. Everything from
hieroglyphics to HTML can be defined using SGML, negating any need for any other markup language.
The problem with SGML is that it is so broad and all-encompassing that mere mortals cannot use it. Using SGML
effectively requires very expensive and complex tools that are completely beyond the scope of regular people who
just want to bang out an HTML document in theirspare time. As a result, HTML and other language standards
adhere to some, but not all SGML standards,[2] eliminating many of the more esoteric features so that HTML is
readily useable and used.
[2] The HTML DTD in Appendix D uses a subset of SGML to define the HTML 4.01 standard.
Recognizing that SGML is unwieldy and not well-suited to describing the very popular HTML in a useful way, and
thatthere was a growing need to define other HTML-like markup languages to handle different network
documents, the W3C defined the Extensible Markup Language (XML). Like SGML, XML is a separate formal
markup metalanguage that uses select features of SGML to define markup languages. It eliminates many features of
SGML that aren't applicable to languages like HTML and simplifies other SGML elementsin order to make them
easier to use and understand.
HTML Version 4.01 is not XML-compliant. Hence, the W3C offers XHTML, a reformulation of HTML to be
compliant under XML. XHTML attempts to support every last nit and feature of HTML 4.01 using the more rigid
rules of XML. It generally succeeds but has enough differences to make life difficult for the standards-conscious
HTML author.
Confused?Don't be. Learning HTML is still the way to go for most authors and Web developers. The native
language endures. Besides, by learning HTML, you learn the working bits of XHTML, effectively the same things.
There are some differences, which we explore in Chapter 16, XHTML. But the differences should not affect your
work in the foreseeable future.
1.5 HTML and XHTML: What They Aren't
With alltheir multimedia-enabling, new page layout features, and the hot technologies that give life to
HTML/XHTML documents over the Internet, it is also important to understand the languages' limitations. They are
not word-processing tools, desktop publishing solutions, or even programming languages. That's because their
fundamental purpose is to define the structure and appearance of documents anddocument families so that they
may be delivered quickly and easily to a user over a network for rendering on a variety of display devices. Jack of
all trades, but master of none, so to speak.
1.5.1 Content Versus Appearance
Before you can fully appreciate the power of the language and begin creating effective documents, you must yield
to one fundamental rule. These markup languages are...