How important is XML? In 1999, the Gartner Group, an Internet research company, said that 80% of business-to-business (also known as B2B ) traffic used XML.
The user and author of a file of XML code can define and use, on-the-fly , brand new command tags that are also available to other Web pages. In defining new XML command tags, you use the extensible portion of XML.
As mentioned in the previous section, HTML describes the format of a Web page; XML describes the content of a Web page. XML does more than just tell the Web browser where and how to place Web page elements.
Consider the following possible XML section from a Web page:
<CARMAKE>Swifty</CARMAKE> <CARMODEL>Dove</CARMODEL> <ENGINEPARTNO>546-32Xs</ENGINEPARTNO> <WHOLESALE>,039</WHOLESALE> <SUGGESTEDRETAIL>,483</SUGGESTEDRETAIL>
Over time, industries will begin to standardize their XML tags. Therefore, <ENGINEPARTNO> might be standardized by automobile Web-site designers to designate any automobile manufacturer's engine part number. As other automobile Web developers standardize and use <ENGINEPARTNO> (and its corresponding ending tag of </ENGINEPARTNO> ), Web pages can be combined, borrowed, and used as the basis of other Web pages that also contain such parts .
When learning XML, you would not learn a <CARMAKE> tag because no such tag exists. As soon as a Web site uses <CARMAKE> , however, the tag is defined and should be used in that context. As a Web designer, you will learn the standard XML tags being used for the type of Web site you are developing. Your XML tags then define data categories, not actual data specifics.
One problem at this time is that XML is too new for globally agreed upon definitions to exist. Even within industries, one company might create XML tags that differ greatly from another's. The move toward organization will come only as companies that support XML begin to agree on a standard and that standard grows. For example, if your company's Web site is to interact with a vendor's XML-based Web site, one of you must adopt the other's XML tags or you must put into place a combined system. This agreement process will continue and grow as more companies move to XML.
HTML has a defined set of formatting and hyperlink tags, and you could very easily learn all of them. XML is defined as Web designers use it. You'll never learn all the XML tags, because new tags will continue to be developed as long as the language is in use.