Case study: The World Wide Web



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The World Wide Web [www.w3.org I, Berners-Lee 1991] is an evolving system for publishing and accessing resources and services across the Internet. Through commonly available web browsers, users retrieve and view documents of many types, listen to audio streams and view video streams, and interact with an unlimited set of services. The Web began life at the European requirement to organize their knowledge. This means that documents contain links (or hyperlinks) – references to other documents and resources that are also stored in the Web. It is fundamental to the user’s experience of the Web that when they encounter a given image or piece of text within a document, this will frequently be accompanied by links to related documents and other resources. The structure of links can be arbitrarily complex and the set of resources that can be added is unlimited – the ‘web’ of links is indeed world-wide.
Bush [1945] conceived of hypertextual structures over 50 years ago; it was with the development of the Internet that this idea could be manifested on a world-wide scale.
The Web is an open system: it can be extended and implemented in new ways without disturbing its existing functionality. First, its operation is based on communication standards and document or content standards that are freely
published and widely implemented. For example, there are many types of browser, each in many cases implemented on several platforms; and there are many implementations of web servers. Any conformant browser can retrieve resources from any conformant server. So users have access to browsers on the majority of the devices that they use, from mobile phones to desktop computers.
Second, the Web is open with respect to the types of resource that can be published and shared on it. At its simplest, a resource on the Web is a web page or some other type of content that can be presented to the user, such as media files and documents in
Portable Document Format. If somebody invents, say, a new image-storage format, then images in this format can immediately be published on the Web. Users require a means of viewing images in this new format, but browsers are designed to accommodate new
content-presentation functionality in the form of ‘helper’ applications and ‘plug-ins’. The Web has moved beyond these simple data resources to encompass services, such as electronic purchasing of goods. It has evolved without changing its basic
architecture. The Web is based on three main standard technological components:
the HyperText Markup Language (HTML), a language for specifying the contents and layout of pages as they are displayed by web browsers;
Uniform Resource Locators (URLs), also known as Uniform Resource Identifiers (URIs), which identify documents and other resources stored as part of the Web;
a client-server system architecture, with standard rules for interaction (the HyperText Transfer Protocol – HTTP) by which browsers and other clients fetch documents and other resources from web servers. Figure shows some web servers, and browsers making requests to them. It is an important feature that users may locate and manage their own web servers anywhere on the Internet.
centre for nuclear research (CERN), Switzerland, in 1989 as a vehicle for exchanging documents between a community of physicists connected by the Internet [Berners-Lee 1999]. A key feature of the Web is that it provides a hypertext structure among the documents that it stores, reflecting the users’ 

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We now discuss these components in turn, and in so doing explain the operation of browsers and web servers when a user fetches web pages and clicks on the links within them.
HTML • The HyperText Markup Language [www.w3.org II] is used to specify the text and images that make up the contents of a
webpage

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, and to specify how they are laid out and formatted for presentation to the user. A web page contains such structured items as headings, paragraphs,tablesand images. HTML is also used to specify links and which resources are associated with them.
Users may produce HTML by hand, using a standard text editor, but they more commonly use an HTML-aware ‘wysiwyg’ editor that generates HTML from a layout that they create graphically. A typical piece of HTML text follows:
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