Overall Summary

Over the course of the last few decades as the web has matured, content has been delivered in different flavors of markup languages. The World-Wide Web Consortium, or W3C, an international group of members and staff dedicated to developing web standards, has helped lead the way in establishing standards for what have been three major markup languages, namely HTML4.0, XHTML, and the still-emerging HTML5 standard. Below is a brief summary of the history and key features of each markup language.

Development of HTML 4.01, and it's distinguishing factors

As stated on the W3C website, HTML was originally developed by Tim Berners-Lee while at CERN, and popularized by the Mosaic browser developed at NCSA, in the early 1990's. By the mid-90's, HTML 2.0 had developed, and by December of 1997, as Land of Code states on their website, "HTML 4.0 was published as a recommendation by the W3C." Because HTML was originally designed to specify the logical organization of a document, HTML 4.0 returned to this focus, by deprecating styling tags and attributes to a separate document, making HTML easy to work with again. The iteration of HTML 4.01 saw improvements in specific selectors, namely the form, frame, and input selectors, among others.
Introduction to HTML 4.0. www.w3c.org

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Development of XHTML, and it's distinguishing factors

XHTML 1.0 specification became a W3C recommendation on January 26, 2000, and was a merger of XML markup rules brought into use like traditional HTML. With this emergence, web developers had to classify their document types as one of three types: transitional, frameset and strict, depending on which features, deprecated or not, they were going to feature in their markup document. It was deemed the savior of a sloppy markup world, bringing much-needed order back to the forefront of document structure.
W3C XHTML Activities

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Development of HTML5, and it's distinguishing factors

In the early 2000's, a rebellion formed within the W3C, chiefly over the lack of backwards-compatibility that the newly proposed XHTML 2.0 spec had established. Some of the key authors formed their own group in 2004, and began a project originally titled Web Apps 1.0, under the group name WHATWG, or Web Hypertext Application Technology Working Group. In 2006, the W3C admitted that their efforts weren't working to improve XHTML, and abandoned the effort formerly in 2009, shifting efforts and working in tandem with WHATWG. HTML5's chief aim is to bring a new form of structures and syntax for handling complex procedures demanded by modern websites, but it's really just the next step in the evolution of markup.
A Brief History of Markup by Jeremy Keith

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Valid HTML5 & CSS