The relationship between SGML, HTML, XML, XHTML

Let's look at their historical origins first:

• sgml--1986 International Organization for Standardization publishes a standard in information management (ISO 8,879:1986 processing).
• HTML 2.0--released as RFC 1866 in November 1995
• XML 1.0--1998, published the XML1.0 specification, used to simplify the transmission of document information on the Internet
• XHTML 1.0--2000 as a recommended standard for the world of
• HTML5--2014, announced that the standard was set up to complete

Note: HTML 1.0 was published as a draft of the Internet Engineering Working Group (IETF) in June 1993, not a standard.

As you can see from the table above, SGML is the oldest, nearly 20 years after SGML became an international standard, HTML 2.0 is called the recommended standard of the world, and the latest development of XML, SGML and HTML is obviously the pioneer of XML.

Let's start with SGML, which is not a format language, not even a specific markup language. SGML is a set of specifications that allows people to create their own markup language. It provides a content qualifier that makes consistent formatting of text very easy, allowing the document management system to quickly position information. SGML is ideal for designing a large number of structurally similar data projects, such as catalogs, manuals, checklists, transcription, and statistical abstracts. It enables developers to assemble together to easily develop a data structure specification, create a document type definition (DTD), and then apply it to documents throughout the organization.

What does SGML have to do with HTML? HTML4 Previous versions (including HTML4) are theoretically based on an instance of SGML, so you need to reference the associated DTD in DOCTYPE. The reason for this is that it is said that the previous version of HTML4 (which does not contain HTML4) is not strictly in accordance with SGML.

What is the relationship between SGML and XML? XML is a subset of SGML, which is simpler than SGML, but can achieve most of the functionality of SGML. The advent of XML is to solve the problem of HTML, such as:

• does not solve all the explanatory data--such as audio-visual files, chemical formulas, musical symbols and other forms of content
• Performance Issues-Download the entire document to search for files
• Scalability, elasticity, legibility are poor

The XHTML developed later is an example of XML-based, and XHTML appears to solve the problem of loose HTML syntax requirements. Today's HTML5 are no longer based on SGML.

The relationship between SGML, HTML, XML, XHTML