Encyclopedia of Computer Science and Technology

hypertext and hypermedia 233guage, including better support for frames, columns, tables,and other formats. Browser developers have also adopteda system that allows document authors to define generalstyles to ensure consistent document appearance (see cascadingstyle sheets). Style sheets can inherit styles fromother style sheets, allowing an organization to create generalstyle sheets that can then be refined to create specializedstyles for particular types of documents. The latestversion of HTML (as of 2007) is 4.01, with 5.0 still in draft.Dynamic HTML (DHTML) is a set of techniques thatallow otherwise fixed (“static”) HTML pages to be changedas users are viewing them. A scripting language (see, forexample, JavaScript) is used to change the specifications(usually via the style sheet). The programming interfaceto the Web page is the document object model (see dom).DHTML can be used, for example, to create drop-downmenus or “rollover” buttons that change as the mouse navigatesover them. Even simple games have been written inDHTML to run in Web browsers. DHTML should be distinguishedfrom other dynamic techniques such as server-sidescripting (see Perl and php), which changes the page beforeit is presented to the user, and asynchronous techniquesthat can change a part of a page without reloading it (seeAjax).XHTML is essentially a rewriting of HTML according tothe syntax of the Extensible HyperText Markup Language(see xml). Because of the stricter syntax rules for XML,XHTML cannot use many of the earlier free-form structuresof HTML. However, because XML has become so prevalenta means for connecting Web pages to data sources, there aremany xmL tools that XHTML authors can use for parsingand syntax checking. As of 2007, XHTML 1.1 is the prevailingstandard, but a draft 2.0 version represents a more thoroughbreak from the elements of the original HTML.Further ReadingFreeman, Eric, and Elisabeth Freeman. Head First HTML with CSSand XHTML. Sebastapol, Calif.: O’Reilly Media, 2005.Goodman, Danny. Dynamic HTML: The Definitive Reference. 3rded. Sebastapol, Calif.: O’Reilly Media, 2006.Lloyd, Ian. Build Your Own Website the Right Way Using HTML &CSS. Lancaster, Calif.: SitePoint, 2006.Musicano, Chuck, and Bill Kennedy. HTML & XHTML: The DefinitiveGuide. 6th ed. Sebastapol, Calif.: O’Reilly Media, 2006.Olsson, Tommy. “Bulletproof HTML: 37 Steps to Perfect Markup.”Sitepoint. Available online. URL: http://www.sitepoint.com/article/html-37-steps-perfect-markup. Accessed August 7, 2007.Tittel, Ed, and Mary Burmeister. HTML 4 For Dummies. 5th ed.Hoboken, N.J.: Wiley, 2005.hypertext and hypermediaMost computer users today are familiar with the conceptof hypertext, even if they don’t often use the term itself.Each time a Web user clicks on a link on a Web page, heor she is using hypertext. Most on-line help systems alsouse hypertext to take the reader from one topic to another,related topic. The term hypermedia acknowledges modernsystems’ use of many kinds of resources other than plaintext, including still images, videos, and sound recordings.In a traditional document, the reader is generallyassumed to proceed sequentially from the beginning to theend. (Although there may well be footnotes or cross-referenceswithin the document, these are generally experiencedas temporary divergences from the primary, sequential narrative.)Generally speaking, each reader might be expectedto acquire roughly the same set of facts from the document.In a hypertext document, however, the links betweentopics create multiple potential paths for readers. To theextent the author has provided links between all relatedtopics, the reader is free to pursue his or her particularinterests rather than being bound by a sequential structureimposed by the author. For example, in a document thatdiscusses various organisms in an ecology and the effects ofclimate and vegetation, one reader might choose to exploreone organism in depth, following links from it to otherresources devoted to that organism (including outside Webpages, images, videos, and so on). Another reader might beinterested specifically in the effects of rainfall on the ecologyas a whole and follow a completely different set of linksto sites having climatological data.History and DevelopmentIn 1945, a time when the very first digital computers werecoming on-line, Vannevar Bush, a pioneer designer of analogcomputers, proposed a mechanism he called the Memex(see Bush, Vannevar). This system would link portions ofdocuments to allow retrieval of related information. Theproposal was impracticable in terms of the very limitedcapacity of computers of the time. By the 1960s, when computershad become more powerful (and the minicomputerwas beginning to be a feasible purchase for libraries andschools), another visionary, Theodore Nelson, coined theterms hypertext and hypermedia. He suggested that networking(a technology then in its infancy) could allow forwhat would eventually amount to a worldwide database ofinterconnected information. Nelson developed his specificationsfor a system he called Xanadu, but he was unable tocreate a working version of the system until the late 1990s.However, in 1968 Douglas Engelbart (also known as theinventor of the computer mouse) demonstrated a more limitedbut workable hypertext system called NLS/Augment.During the 1970s and 1980s, a variety of hypertext systemswere created for various platforms, including Guideand Toolbook for MS-DOS and Windows PCs. Perhapsthe most influential system was Hypercard, developed forApple’s Macintosh. While Hypercard did not have a completeset of facilities for creating hypertext, the flexible,programmable, linkable “cards” could be used to implementhypertext documents. Many encyclopedias and otherreference products on CD-ROM began to implement someform of hypertext links.The true explosion of hypertext came with the developmentand growth of the World Wide Web throughout the1990s. Hypertext on the Web is implemented through theuse of HTTP (HyperText Transport Protocol) over the Internet’sTCP/IP protocol and by coding documents in HTML(Hypertext Markup Language). (See html, Internet, tcp/ip, and World Wide Web.)