One of the requirements for creating a usable, nonlinear information system like HTML is to make the system easy for non experts to understand and use. Berners Lee and his colleagues used SGML to describe the rules for hypertext markup language in a document type definition (DTD), the basis for the structure of documents on the World Wide Web.

Level 0 HTML : At level 0, HTML offered a platform-independent means of marking data for interchange. The concept was that servers would store and supply data and clients would retrive and display it.

Level 1 HTML : The idea of a HTML container was added, with a HEAD element seprate from the Body element. Opening and Closing tags were required for some elements.

HTML+ : HTML+ incorporated graphical and display elements into HTML. Elements for superscripts and subscripts, footnotes, margins, inserted and deleted text.

HTMl 2 : Level 2 added the FORM element with INPUT, SELECT, OPTION, and TEXTAREA plus the BR element for line breaks. It also added the META element for detailed document description, which also provided an avenue for indexing and cataloging the contents; also changed the description of the head and body section.

HTML 3 : HTML 3 included a FIG element that supported text flow around figures; ALIGN attributes that enabled left, right or center justification; additional attributes for background images, tabs, footnotes and banners.
HTML started fading away with the advent of Microsoft's Internet Explorer and Netscape Navigator.

HTML 3.2 : It added the SCRIPT and Styles tags. It offered new elements and attributes that enlivened Web pages with animation, colors and sound.

HTML 4 : The latest version of HTML enables seprating physical styles from the content markup by relying on style sheets; introduces the OBJECTS element; included the STYLE, DIV and SPAN elements for incorporating style sheets.