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Computers use a set of programing scripting and markup languages to perform many tasks and functions. The language most computers find easy to understand are Basic C and Java which are compiled powerful and complex languages.
Html/Xhtml are more simple computer language mostly used to markup the structure of complex text documents and most webpages for use on a local intranet or the WWW on the Internet. The Computer also uses a variety of technology oriented scripting and programing languages like Javascript, CGI, VBScript, ActiveX, and PHP that perform allot of the interactive and dynamic functions on most computers.
Standard Generalized Markup Language
(Standard Generalized Markup Language) SGML is a precursor of (GML) General Markup Language. GML was introduced in the 1960s by Charles Gold-fab, Raymond Lari and Edward Mosher working at IBM. The purpose of Sgml was to help organize the large amount of data and documentation that the company was producing. Html, Xhtml and Xml are all children of standard generalized markup language.
XML Meta Language Application
XML (Extensible Markup Language) was created as a subset of Sgml. It's actually a lighter or modified version of SGML. Today, Xml is the new standard that any markup language must adhere to. It is a general markup language, or, we can call it "the Father of all languages" that can be used to describe other markup languages including Html.
Xml is not entirely about webpages, it is much more than the web. Webpages written in Xhtml can easily interact with Xml which can be used to represent any kind of information on all types of computers, hand held devices and cellphones.
HTML Introduction To The Web
In 1986, Tim Berners-Lee created (HTML) Hypertext Markup Language as a subset of SGML by using the rules of Sgml to create the tags used in html documents. The original purpose of Html was to transfer and display scientific data on local intranet. It had a definite structure that allowed the scientific community to view, reference and easily share information on separate computers via Hyper text transfer protocol.
Hypertext originally meant text stored in electronic format with cross-reference links between documents. Today it is used in a broader capacity to link to almost anything, including videos, images, audio and other objects on the web.
XHTML Extensible Markup Standard
XHTML is more than Html and there are some basic difference in writing Xhtml and Html. Xhtml is an extensible markup language that uses the syntax and rules of XML. This makes XHTML far more restrictive than Html, for it does not allow you to bend or break the (syntax) rules governing the document markup method and its specifications.
HTML5 Official Web Markup Standard
Html5 is the official markup language standard for building standard compliant websites.
HTML Document Types
A (DTD) Doctype is used to declare the type of Html or Xhtml markup the Web author is supposedly using in the document. HTML 4.01 specifies Three Doctype Definitions. Each vary in the elements they support, so web authors must include one of the following document type declarations in their Html documents (Webpages). The new Html5 has one Doctype declaration, and It's the official doctype used to for building websites today
Markup Language Validity & Conformance
Markup languages are defined in technical specifications, which generally include a formal grammar. A document is valid when it is correctly written in accordance to the formal grammar, whereas conformance relates to the specification itself. The two might be equivalent, but in most cases, some conformance requirements cannot be expressed in the grammar, making validity only a part of the conformance.