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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)                 9  TCP/IP World Wide Web (WWW, "The Web") and the Hypertext Transfer Protocol (HTTP)                      9  TCP/IP World Wide Web and Hypertext Overview and Concepts

World Wide Web System Concepts and Components 1 2 3 4 World Wide Web Addressing: HTTP Uniform Resource Locators (URLs)

We've seen in the last two topics that the World Wide Web is based around the central concept of hypertext. The prefix “hyper” usually means “above” or “beyond”, and thus hypertext is like text, but goes beyond it in terms of functionality. Documents written in hypertext are similar to regular text files, but include information that implements hypertext functions. These are, of course, usually called hypertext documents or hypertext files.

The extra information in a hypertext document is used to tell the computer program that displays the file to a user how to format it. This information takes the form of special instructions that are interspersed with the actual text of the document itself, which are written according to the syntax of a defining language. This addition of extra elements to the content of a document is commonly called marking up the document.

The standard language used by the World Wide Web is thus called the Hypertext Markup Language (HTML). HTML is one of the three primary system components of the World Wide Web, and was invented in 1990 by the creator of the Web, Tim Berners-Lee. It was not created in a vacuum; rather, it is a specific application of the general concept of a markup language that is described in ISO standard 8879:1986: Standard Generalized Markup Language (SGML).

The idea behind a markup language is to define special items that provide information to the software displaying the document about how it should be presented. For the purposes of hypertext, the most basic type of information in a document is a special instruction that specifies how one document can be linked to another—after all, this linking process is the defining attribute of hypertext.

However, HTML goes far beyond just this; it defines a full set of text codes for describing nearly every aspect of how a document is shown to a user. This includes instructions for formatting text (such as defining its color, size and alignment), methods for displaying tabular data, specifications for how to present images and other media along with the document, interactive forms and much more. In theory, the language is only supposed to define the document and leave up to the browser how it should be displayed, but in practice, modern HTML documents also usually contain rather specific instructions for how their information should be presented.

To do justice to HTML, I would have to devote several dozen pages to the subject. I have decided not to do this, because even though HTML is an important part of the Web, it is actually not that important in understanding how the Web works. Knowing HTML is essential if you are writing Web content, and is also critical if you want to understand how to write Web software. Perhaps ironically though, to the actual mechanisms that make the Web work, such as HTTP, “a document is a document”. HTTP is not designed under the assumption that it will transfer HTML, and servers in most cases do not even look at the contents of an HTML file—they just transfer it.

That said, a basic understanding of HTML is important, and it just wouldn't seem right not to provide at least an overview of the language, so I will do that here. I encourage you to seek out one of the many good HTML resources if you want to learn more: there are dozens of them on the World Wide Web (where else? J)

World Wide Web System Concepts and Components 1 2 3 4 World Wide Web Addressing: HTTP Uniform Resource Locators (URLs)