HTML4 became the standard for hypertext documents on the web in 1997. The idea behind HTML4 was to provide bare-bones markup focused on content, and to push content-providers into doing their procedural markup in Cascading Style Sheets. HTML4 was largely successful in this regard. At the time the feeling at the W3C was that HTML was nearing the end of its usefulness, and would be superceded by XML. To that end development for HTML focused on XHTML.
In the early part of this century, a few flaws in this strategy became apparent. First, use of HTML persisted, in spite of improving tools and support for XHTML, and even those who moved toward XML largely used XSLT to render it into HTML. Moreover, many new media became prevalent on line, including vector graphics audio, and video, and there was a need for a unifying framework in which to display these. In 2004 work was begun on a new standard for HTML, and later work was stopped on a new XHTML standard. The work on HTML is coming to maturity as the HTML5 standard.
HTML5 incorporates a number of new features, including new contextual tags, and some support for audio and video formats. The most significant novelties for this course in HTML5 are that it incorporates both MathML and SVG into it. This reduces the technical difficulty of building and supporting mathematical documents on the World Wide Web. As of this writing the current versions of most browsers support a version of HTML5.