Web pages are displayed in an application called a browser. Internet Explorer, Netscape, Mozilla, and Opera are some of the many browsers available. Besides displaying the page, the browser is responsible for retrieving it, usually across the Internet. The browser must also sometimes send user information back across the Internet, as happens when a user has filled out a form.
Browsers also contain utility functions to improve the user's experience, such as maintaining a history of recently viewed pages. To improve performance, most browsers also have a cache. CPUs have caches to keep recently used data nearby so that a request doesn't have to be made to main memory if the data is needed again soon. A Web cache employs the same principle, keeping a copy of recently viewed web pages on the user's hard drive and displaying the copy of a page when the user requests to view it, rather than waiting for it to cross the Internet again.
Still, the most important thing the browser does is display the page. In a perfect world, all browsers would display the same page in exactly the same way. Unfortunately, this is not the case. Early in the history of the Web and HTML, the companies developing browsers were in a hurry to introduce new features, moving faster than the organization responsible for developing HTML itself. They created their own flavors of HTML (adding their own tags), and this tactic encouraged designers to create pages that wouldn't display properly except in that company's browser.
When the Web and HTML were first gaining momentum, a debate raged over how specific the markup tags should be. Essentially, the debate was between those who thought the tags should specify as much as possible and those who thought they should be more like guidelines.
For example, rather than using a <i> tag for italics, those in the latter camp preferred to use an <em> tag, which is short for "emphasis." In most browsers, text marked with <em> would still display in italics, but the browser could render it in boldface, or in a bright red color, or any other way that would clearly emphasize the text. A tag like <em> is a logical tag, which describes how text relates to the document, rather than how it actually appears.
It appears that the "exact layout" camp won out over the "logical guidelines" camp. For graphic designers, exact layout is a good thing, because they have more precise control over the appearance of each page, but for some users this choice is a bad thing. Exact specifications cause problems with accessibility, which refers to ease of use for people with disabilities. For example, a designer who uses tags to highlight important blocks of text in a different color isn't helping a viewer with color blindness.
The situation isn't quite as bad now. It's rare to find a web page that displays properly in one browser but is unreadable in another. Still, some differences remain, and they're enough to make a good layout turn ugly. Sometimes these differences can exist even between older and newer versions of the same browser.
These differences leave the designer two choices. One choice is to declare that the web page has been designed for a particular browser and that any problems with other browsers are the user's problem. The other choice is to check each page in a cross part of popular browsers and avoid tags and designs that are known to cause problems.
Some Web sites have pages that change so frequently, it's not feasible to run each one through an HTML editor when the changes are necessary. A dynamic web page is a web page that is generated when the user requests it, rather than created once and subsequently merely copied to the user. Dynamic web pages allow form to be separated from content even more than style sheets do.
For example, in a news organization, a template for a page can be created that essentially says, "Main story goes here," "Top sports story goes here," "Advertisement goes here," and so on. When the user requests to view the page, the Web site's server fills in the template with the current stories, a randomly selected advertisement, and everything else.
Dynamic web pages are an example of server side scripting, which is programming performed on a Web server using a scripting language. Client side scripting, in contrast, embeds small pieces of programming code in a web page, which is run on a user's machine in response to user interaction. When you fill out a form on a Web site and click the "Submit" button, you are executing a piece of client side scripting.
Server side scripting is also used for handling user data once it reaches the server. For example, a Web site for paying an electric bill may request that the user enter an account number. The server side scripting can check with the utility company's database to determine if the account number is valid and, if so, retrieve the account data, at which point it generates a dynamic web page to return to the user.
Client side scripting can perform only basic tasks. To execute a server side script requires a "round trip"; that is, data must travel from the user's computer to the Web server and back again, which takes time. For faster and richer user interaction, some designers include embedded programs that run on the user's computer inside the web page.
Two common mechanisms for providing embedded programs are the Java programming language, which allows the creation of "applets," - small programs with full graphical interfaces - and Macromedia's Flash software, which allows nonprogrammers to create animations and interactive programs in a style that is well suited to tutorials.
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Note: This article was sent to us by: Nelson G. Hill at 02132011
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