Extensibility compared with HTML

I believe that more people are more familiar with XML. Just open a web page and right-click to view the source file. Then The of presented inside is the HTML code that provides us with data and styles. This article briefly describes the differences between HTML and XML, which are also markup languages.

Let’s start with the name analysis. I believe that readers can understand the reason why XML is an extensible markup language through the following two examples.

HTML code

<html><!--这是HTML文档--><head><title>学习XML</title></head><body>你好,XML</body></html>

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XML code

<?xml version="1.0" encoding="gb2312"><!--这是XML文档--><可扩展标记语言><题目>学习XML</题目><内容>你好,XML</内容></可扩展标记语言>

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It can be seen from the above example that the various tags in the HTML document are a set of predetermined tags, while the tags in the XML document are specified by the user themselves, as long as they comply with the XML specification (in This will be explained in detail in a later article) Users can define their own tags at will.

As introduced in the previous article, SGML is a universal language that uses markup to describe document data that has existed long before the invention of the Web. In the Web, due to the huge popularity of SGML, people began to use XML instead of SGML. But it is undeniable that HTML and XML are both markup languages developed from SGML. Therefore, they have things in common, such as similar syntax and use of markup. However, HTML is a descriptive language under the definition of SGML, and is just an application of SGML. Its DTD (the DTD will be described in detail in the next article, here the reader only needs to know that the DTD is related to the number of tags) as Standards were fixed.

The

HTML tag is fixed so it is difficult to define new applications. "The starting point of XML's design is to take the advantages of SGML, remove its complex parts, and keep it lightweight and able to work on the Web"----Tim Bray. Therefore, XML is a simplified version of SGML and a subset of SGML. In a strict sense, XML is still SGML, and DTD can be flexibly defined for it.

It is precisely because of the flexible extensibility of XML that it has been used in many aspects. For example, in Ajax, the XMLHttpResponse object returns XML in most cases, and in JavaWeb development, most configuration files are also stored in the form of XML, and in EJB When using the WEB method for remote calling, the first thing to read is XML (although it is a file in the form of wsdl, its essence is also XML) and so on. These all take advantage of the extensibility of XML to effectively store data.

The above is the content of XML Concise Tutorial (2). For more related content, please pay attention to the PHP Chinese website (www.php.cn)!