HTML and CSS Reference
The World Wide Web is composed of files containing Hypertext Markup Language
(HTML) and other markup languages that describe Web pages. HTML was developed
using Standard Generalized Markup Language (SGML). SGML prescribes a standard
format for embedding descriptive markup within a document and for describing the
structure of a document. SGML is not in itself a document language, but rather a
description of how to specify one and create a document type definition (DTD) .
The W3C, http://w3c.org, sets the standards for HTML and its related languages.
HTML (like the Web itself) is in a constant state of change.
HTML is the set of markup symbols or codes placed in a file intended for display on a
Web browser page. These markup symbols and codes identify structural elements such
as paragraphs, headings, and lists. HTML can also be used to place media (such as
graphics, video, and audio) on a Web page and describe fill-in forms. The browser
interprets the markup code and renders the page. HTML permits the platform-
independent display of information across a network. That is, no matter what type of
computer a Web page was created on, any browser running on any operating system
can display the page.
Each individual markup code is referred to as an element or tag . Each tag has a pur-
pose. Tags are enclosed in angle brackets, the < and > symbols. Most tags come in pairs:
an opening tag and a closing tag. These tags act as containers and are sometimes
referred to as container tags. For example, the text that is between the <title> and
</title> tags on a Web page would display in the title bar on the browser window.
Some tags are used alone and are not part of a pair. For example, a tag that displays a
horizontal line on a Web page, <hr /> , is a stand-alone or self-contained tag and does
not have a closing tag. You will become familiar with these as you use them. Most tags
can be modified with attributes that further describe their purpose.
The most recent standardized version of HTML used today is actually eXtensible
HyperText Markup Language (XHTML) . XHTML uses the tags and attributes of
HTML 4 along with the syntax of XML. HTML was originally developed to provide
access to electronic documents via a Web browser. Web browsers that evolved along
with HTML were written to forgive coding errors, ignore syntax errors, and allow
“sloppy” HTML code. Web browsers contain many program instructions that are
designed to ignore mistakes such as missing ending tags and to guess how the developer
meant the page to display. This is not a problem for a personal computer, which has
relatively large processing power. However, this could be an issue for electronic devices
with fewer resources, such as a personal digital assistant (PDA) or mobile phone.
Also, as new versions of Web browsers were developed and competed for market share,
they often created their own proprietary extensions to HTML—tags that were not part
of the standard and supported by one browser only. This created a lot of nonstandard
HTML pages, and browsers are coded to accept this and ignore tags they don't recog-