Tags' Symbols and Rules, Markup Languages

Do you know that the first computer language for developing websites that a web developer should learn is a markup language? It is used to markup a document, have symbols and rules for doing elements and attributes of a document, and that's why it is the most used type of computer language on the World Wide Web (simply as WWW or W3,) because without this, you can't markup correctly, such as no link, no table, no title, no description, and no keyword, just a graphical text.

A markup language that have an application, can be called as a "generalized markup". The most used generalized markup in this time is listed below:

• A Standard Generalized Markup Language (SGML for short,) is developed and standardized by the International Organization for Standardization (or ISO) in 1986. Most tags of a SGML does have a start tag and end tag, same to the tags of the SGML standard and SGML-based languages' HTML and XML standard. SGML was extended from Generalized Markup Language (GML). SGML and GML are both systems for organizing and tagging elements, but SGML wasn't based on GML, they are completely different! Because GML has different symbols such ordered list's start :ol tag and end :eol. tag, and most of GML tags doesn't have an end tag such :li. for defining list item, :h1. to define heading 1 and :p. to define a paragraph.

There are many SGML-based languages or applications of a Standard Generalized Markup Language exist now and the most used of those languages are listed below:

1. Hyper Text Markup Language (also known as HTML,) is have their own set of predefined tags for doing elements and attributes of a document. .html and .htm are the file name extensions for HTML files. The HTML tags that has no end tags can written as just their start tag such <br> without closing it and can be closed as <br/>, all HTML tags are case-insensitive and even the format that you've been made wasn't right, most web browsers will display it correctly. And because of these, HTML doesn't have application.
2. XML means Extensible Markup Language, originally designed to carry data and have an .xml filename extension. In XML, all elements must properly closed and the tags' capitalization are case-sensitive. The XML standard tags are not predefined as HTML, so you could invent your own tag such start <invented> tag and end </invented> tag, this tag was just invented. There are now many XML-based languages exist because of its proper and well-organized syntax, including the popular XHTML.
   The often used families of markup languages of Extensible Markup Language, also known as XML-based languages or applications, are listed below:
   1. XHTML means Extensible Hyper Text Markup Language, it has the same set of functions and tags as HTML, but with syntax as strict and clean as XML, such a tag that has no end tag in HTML such <br> or <br/> must be properly closed as <br /> and tags' capitalization are case-sensitive like XML, and that's why it's in the family of XML instead of HTML. The filename extensions that build for XHTML files are .xhtml and .xht but XHTML can also use the XML standard and HTML's filename extensions. Yet, the .html filename extension is still the most used.
   2. A computer language, Atom (standard) is used for developing web feeds. Atom documents has a filename extension of .atom, or .xml as it is an aplication of Extensible Markup Language. An HTML document can be linked to an ATOM feed using <link> tag. Atom is more advanced syndication format than RSS.
   3. Rich Site Summary also known as Really Simple Syndication (abbreviated as RSS,) is a family of news feed formats and their documents have .rss or .xml filename extension. The first version of RSS was released in March 1999, it is older than Atom standard which was first published in December 2005, and a former primary method of web content syndication before Atom.