Chapter 16

1. Chapter 16 The World Wide Web

2. The World Wide Web The World Wide Web (Web) is an infrastructure of distributed information combined with software that uses networks as a vehicle to exchange that information. The Internet makes the communication possible, but the Web makes that communication easier, more productive, and more enjoyable.

3. The World Wide Web A Web page is a document that contains or references various kinds of data, such as text, images, graphics, and programs. Web pages also contain links to other Web pages so that the user can “move around.” It’s these links that turn text documents into hypertext. A Web site is a collection of related Web pages, connected by links.

4. Web Browser A browseris a software tool that acts as a network client. It issues requests for the Web pages we want and displays them when they arrive. We often talk about “visiting” a Web site, as if we were going there. In truth, we request the information we want, and it is served to us.

5. Web Browser

6. URLs The request sent by the browser is called a Uniform Resource Locator, or URL. It uniquely identifies the page you want among all of the files stored anywhere in the world accessible to the Internet. This identification contains the file’s name and location, as well as the set of rules required to transfer it.

7. URLs Every URL has four parts: protocol http is the protocol used to transfer Web pages hostname path filename E.g. http://www.eecs.yorku.ca/course/1520/index.html I. II. III. IV.

8. Search Engines Search Engine: A website to find other websites. Examples include: AIO Search, Ask Jeeves, Bing, Blippex, Google, kifi, Recusive, Omni, Search All, Search Me, Spotify, Yahoo Each maintains a database of information, including an index of keywords and URLs that feature them. The dBase is maintained by “spider bots” that “crawl” the web, examining more than 1 trillion (1,000,000,000,000) unique URLs.

9. Search Engines When you enter keywords, the search engine produces a list of links to potentially useful sites. Of course, ‘useful’ means different things! There are two approaches to searching: • Keyword searches • Concept-based searches

10. Instant Messaging Instant messaging (IM): An application that allows people to send and receive messages in real time. Both sender and receiver must have an IM app running. Most IM applications use a proprietary protocol that dictates the precise format and structure of the messages that are sent across the network to the receiver. Instant messages are not secure! Except BBM

11. Blogs • Short for weblog • Publishing mechanism • Use has evolved • “citizen journalism”

12. Cookies Cookie: A small text file that a web server stores on your local computer’s hard disk. A cookie contains information about your visit to the site. Cookies can be used… to determine number of unique visitors to the site, to customize the site for your future visits, to implement shopping carts that can be maintained from visit to visit. Cookies are not dangerous!

13. HTML Web pages are created (or built) using a language called the Hypertext Markup Language, or HTML. The term markup language comes from the fact that the primary elements of the language take the form of tagsthat we insert into a standard text file to annotate the information stored there.

14. HTML • Since web pages are standard text files they can be created with any text editor. • They can also be transferred very quickly. • The files are saved with the file extension .html (or .htm) so browsers will apply the appropriate rules to the text.

15. HTML – a brief history • 1980 - Tim Berners-Lee began development of HTML at CERN . • 1991 - First specification for HTML published. • 1995 - Request for Comments 1866 specifies HTML 2.0. • 1996 - World Wide Web Consortium (W3C) maintains specifications for HTML. • 1999 - HTML 4.01 published. • 2004 - development began on HTML5.

16. HTML – the future • 2011 - HTML5 standard released • HTML5 responds to the regret of web designers over the conflation of Structure and Presentation in HTML. • Itlooks very promising for serious developers, providing a streamlined tag system and support for dynamic content.

17. XHTML – the present • HTML5 is not yet widely supported by browsers, but the important shift in design philosophy is embodied in XHTML. • The current style is to use XHTML to specify Structure, and Cascading Style Sheets (CSS) to specify Presentation.

18. What Is XHTML? • XHTML - eXtensible HyperText Markup Language • very similar to HTML 4.01 • stricter and cleaner • HTML defined as an XML application • a W3C Recommendation of January 2000 • supported by all major browsers

19. Structure of an XHTML document An XHTML document consists of several parts: • the <!DOCTYPE… > declaration • the <head> section • the <body> section

20. Structure of an XHTML document <!DOCTYPE …> <html> <head> <title>appears in the Title Bar</title> </head> <body> … </body> </html>

21. XHTML • The <!DOCTYPE> declaration is mandatory in XHTML. • This declaration refers to a Document Type Definition (DTD) which specifies the rules for the markup language, so that the browsers render the content correctly. • For example: • <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" • "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

22. XHTML - Different Doctypes • XHTML 1.0 Strict • This DTD contains all HTML elements and attributes, but does NOT INCLUDE presentational or deprecated elements (like font). Framesets are not allowed. The markup must also be written as well-formed XML. • XHTML 1.0 Transitional • This DTD contains all HTML elements and attributes, INCLUDING presentational and deprecated elements (like font). Framesets are not allowed. The markup must also be written as well-formed XML. • XHTML 1.0 Frameset • This DTD is equal to XHTML 1.0 Transitional, but allows the use of Framesets. • XHTML 1.1 • This DTD is equal to XHTML 1.0 Strict, but allows you to add modules (for example to provide ruby support for East-Asian languages).

23. XHTML • Some important differences between HTML and XHTML: • XHTML elements must be properly nested. • XHTML elements must always be closed. • XHTML elements must be in lowercase. • XHTML documents must have one root element.

24. XHTML - Tags A tag consists of a keyword enclosed in angle brackets. (<. . . >) Tags are most often used in pairs, with a start tag such as <body> and an end tag such as </body> surrounding the text to which they apply. A tag pair, along with its enclosed text, is called an element.

25. A sample document This is the file basic.txt It contains “raw” text and XHTML mark-ups. Tags are <blue>. <html> <head> <title>EECS 1520</title> </head> <body> <h1> Welcome to the Website for EECS 1520 </h1> <p> This is our first foray onto the World Wide Web. We are trying to learn how to create pages that can become part of the WWW. We hope to learn about elements of Structure, including: - headings - paragraphs - tables </p> </body> </html>

26. XHTML The same file displayed in a browser.

27. XHTML Note that the browser determines how the page should be displayed based on the tags. It ignores the way we format the XHTML source document. Tabs, carriage returns, extra spaces, and blank lines are called “whitespace” and are ignored by the browser. The browser takes into account the width and height of its window. When you resize the browser window, the contents of the Web page are reformatted to fit the new size.

28. The body element in the sample page contains 2 other elements: <h1>…</h1> is a heading element. <p>…</p> is a paragraph element. <body> <h1> Welcome to the Website for EECS 1520 </h1> <p> This is our first foray onto the World Wide Web. We are trying to learn how to create pages that can become part of the WWW. We hope to learn about elements of Structure, including: - headings - paragraphs - lists - tables We also hope to learn about elements of Presentation, like: - fonts - colors - formats - pictures </p> </body> Basic Markup in XHTML 28

29. Basic Markup in XHTML Headings in a document. There are six predefined heading levels in XHTML: h1, h2, h3, h4, h5, and h6. The heading level determines the typeface and size of the text, and the vertical spacing below the text. Paragraphs (<p> . . . </p>) specify text that should be treated as a single paragraph. A paragraph always starts on a new line. Paragraphs are followed by vertical spacing. 29

30. Basic Markup in XHTML To cause text to start on a new line without vertical spacing, use the line break element: <br/> This empty element uses a single tag instead of a pair. To conform to the rule that all tags must be closed, the “ /” (space slash) are added to empty tags. 30

31. Lists in XHTML We often want to display a list of items. The <ul>…</ul> pair creates an unorderedlist, the <ol>…</ol> pair creates an orderedlist, and the <li>…</li> pair distinguishes each list item.

32. A sample list <ul> <li>a list item</li> <li>another list item</li> <li>and another list item</li> </ul> This XHTML code will produce…

33. A sample list • …this list in the browser.

34. Tables In XHTML, tabular data can be displayed by using a table comprised of rows of details. <table>…</table> tags surround the table <tr>…</tr> tags surround a row <td>…</td> tags define each detail (data)

35. Tables <tablesummary=“Sample Table”> <tr> <td>col1 row1</td> <td>col2 row1</td> </tr> <tr> <td>col1 row2</td> <td>col2 row2</td> </tr> </table>

36. Tables XHTML requires that web pages be compatible with non-visual browsers. In the case of tables this means that a <table> tag must include a summaryattribute. <table summary=“Brief Description”> The text in the summary provides a description of the table to users of non-visual browsers. 36

37. Tables The sample table as displayed in a browser.

38. Images An image can be incorporated into a Web page using the <img/> tag. This is another example of an empty element since it does not surround text, but instead indicates a point at which to insert the picture.

39. Images To specify where to find the picture to be inserted, the <img/> tag uses the mandatorysrcattribute. <imgsrc=“path/filname.gif” alt=“alternate text” /> Note the other mandatory attribute - alt, which provides text to be displayed whenever the image cannot be. 39

40. Tag Attributes Note the syntax of tag attributes. The attribute name is followed by the equals sign (=), and then the attribute value, which is enclosed in quotes. Either double or single quotes may be used.

41. Links A link is specified using the element a, which stands for anchor. The tag includes an attribute called href that specifies the URL of the destination document.

42. Links For example: <ahref = "http://www.eecs.yorku.ca/course/1520"> EECS1520 Web Site</a> Notes: The URL is enclosed in quotes. The link text between the tags is displayed in the browser, usually in underlined blue. However, this can be changed.

43. Cascading Style Sheets CSS

44. CSS Style Rules • Style Rules tell the browser how a particular part of a web page is to be presented. • Presentation refers to aspects of the display related to the text, like font, colour, size, etc. • For example: • body {background-color: yellow;} • h1 {color: blue;} • p {font-family: verdana; font-size: smaller;}

45. Applying Styles • There are 3 levels at which style rules can be set : • External level • Styles are saved in a separate file (.css) • The XHTML document links to the style sheet. • Document level • Styles are grouped within a <style>…</style> element, which is placed in the <head> element. • Inline level • Style rules are set as the value of the style attribute of a specific element.

46. External level • If our example styles are saved in YandB.css, then we can link it to an XHTML document this way: • <linkrel=“stylesheet” type=“text/css” href=“YandB.css” /> • The document will now have a yellow background, level 1 headings will have blue text, and paragraphs will use a Verdana font, one decrement smaller than normal.

47. Document level • Another way to apply styles to a document is to include them in the head. • <head> • <title>CSE 1520</title> • <style> • body {background-color: yellow;} • h1 {color: blue;} • p {font-family: verdana; font-size: smaller;} • </style> • </head>

48. Inline level • <body style= “background-color: yellow;”> • <h1 style= “color: blue;”>Welcome to the Website for CSE 1520</h1> • <p style= “font-family: verdana; font-size: smaller;”> • This is our first foray onto the World Wide Web. <br /> • We are trying to learn how to create pages that • can become part of the WWW. </p> • We hope to learn about elements of Structure, including: • <ul> • <li>headings </li> • <li>paragraphs </li> • <li>tables </li> • </ul> • </body>

49. Cascading Styles • The more local a style is defined, the higher is its priority. • An External style applies to any document that links to the .css file. • However, a style defined at the Document level will be replace an External style. • Moreover, an Inline style will take precedence over either of the other two. • This is why they’re called “cascading style sheets”.

50. Applying Style Sheets • The advantages of applying style sheets at the External level, make it the current favourite. • Styles can be applied to the whole, or part(s) of a website without having to copy/paste them to multiple documents. • Modifying the presentation of the whole site, or any part of it, requires changes to only 1 file.