Encyclopedia of Computer Science and Technology

72 cascading style sheetsMojave Desert course, navigating by means of a camera, laserrange finders, and radar. In 2007 the contest entered a moredifficult arena, where the robot vehicles had to deal withsimulated urban traffic, negotiate intersections and trafficcircles, and merge with traffic, all while obeying traffic laws.Meanwhile efforts continue for developing a practicalautomated system that could be used for everyday driving. A“tethered” system using magnetic or radio frequency guidesembedded in the road would reduce the complexity of theon-board navigation system, but would probably require dedicatedroads. A “free” system linked only wirelessly would bemuch more flexible, but would require the ability to visualizeand assess a constantly changing environment and, ifnecessary, make split-second decisions to avoid accidents.Such systems may also feature extensive automatic communication,where cars can provide each other with informationabout road conditions as well as their intended maneuvers.The biggest obstacles to implementation of a fully automatedhighway system may be human rather than technical:the cost of the infrastructure, the need to convince thepublic the system is safe and reliable, and concerns aboutpotential legal liability.Ironically, just as information technology is making carssafer, such activities as cell phone use, text messaging, anduse of in-car entertainment systems seem to be makingdrivers more distracted. Whether cars will get smart fastenough to compensate for increasingly inattentive driversremains an open question.Further ReadingDARPA Grand Challenge. Available online. URL: http://www.darpa.mil/grandchallenge/index.asp. Accessed May 18, 2007.Edwards, John. “Robotic Cars Get Street Smart.” Electronic Design55 (June 29, 2007): 89 ff.Shladover, Steven E. “What if Cars Could Drive Themselves?” Availableonline. URL: http://faculty.washington.edu/jbs/itrans/ahspath.htm. Accessed May 18, 2007.Whelan, Richard. Smart Highways, Smart Cars. Boston: ArtechHouse, 1995.cascading style sheets (CSS)Most word processor users are familiar with the use of stylesin formatting text. Using a built-in style or defining one’sown, particular characteristics can be assigned to the structuralparts of a document, such as headings, lead and bodyparagraphs, quotations, references, and so on. There are severaladvantages to using styles. Once a style is associatedwith an element, the formatting attached to that style canautomatically be applied to all instances of the element. If thewriter decides that, for example, level two headings shouldbe in italics rather than normal font, a simple change to the“head2” style will change all level two headings to italics.Cascading style sheets (CSS) extend this idea to thecreation of Web pages. The style sheet defines the structuralelements of the document and applies the desired formatting.Instead of the main text of the document being filledwith formatting directives (see html), a style sheet is associatedwith the document. When a compatible Web browserloads the page, it also loads the associated style sheet andCascading Style sheets enable the appearance and formatting of aWeb page to be handled separately from the page contents. Specificationsprovided in one sheet can be inherited or modified by othersheets.uses it to determine how the page will be displayed. Inother words, the structure of the document is separatedfrom the details of its presentation. This not only makesit easier to change styles (as with word processing), but italso means that different style sheets can be used to tailorthe document to different viewing situations (for example,viewing in a browser on a handheld PDA).CSS uses a standard “box model” for laying out the presentationof a page. From outside in, the areas are definedas outer edge, margin, border, padding, inner edge, and thecontent area. Styles are applied in an order that dependson the relationship of the affected elements. For example,a style defined for the text body will be inherited by theparagraph, which can then redefine one or more of its elements.Similarly, an emphasis style used within a sentencemight override the paragraph style in turn. It is this flowingof definitions down through the hierarchy of styles that createsthe “cascading” part of CSS.As CSS developed further, separate specifications havebeen provided for different media that can be included ina Web page: speech (to be read by a speech synthesizer),Braille (for a tactile Braille system), Emboss (for Brailleprinting), Handheld (for PDAs and other devices with limiteddisplay space), Print, Projection (for computer projectionor transparencies), Screen, Tty (teletype-like displayswith fixed-width characters), and TV.Further Reading“CSS From the Ground Up.” Web Page Design. Available online.URL: http://www.wpdfd.com/editorial/basics/index.html.Accessed May 19, 2007.Lie, Hakon Wium, and Bert Ros. Cascading Style Sheets: Designingfor the Web. 3rd ed. Addison-Wesley Professional, 2005.