Before the foundation of the World Wide Web, encoding of mathematical documents was already a widespread practice. Back in the days when computers were starting to become popular, the ASCII character set (and encodings based on it) was the only widely available encoding scheme. The restrictions of such a limited symbol set were soon apparent.
In the mid seventies, Donald Knuth developed TEX , from which variants such as LATEX stemmed. Layout and typesetting of mathematics is extremely demanding and until now, Donald Knuth's TEX had been able to address these difficulties in a successful way, appealing to the scientific community who has now made it a standard in scientific publishing. TEX has become the tool of choice for producing scientific and mathematical documents.
Despite its widespread use and ease with which it is authored, TEX does not preserve mathematical semantic value, making it unpractical for use in web documents and useless for transmission between applications. TEX is only concerned with describing the presentation of mathematics, not the content. Because people are interested in transmitting their ideas and research via e-mail or web pages it is fundamental that semantic value is kept.
While TEX is mainly a UNIX based application, PC applications dealing with mathematical encoding have also emerged. Generally these are equipped with a graphical user interface making them easier to use: Design Science's MS Word Equation Editor, FrameMaker, WordPerfect or ScientificWord are a few to name examples. All these applications just deal with displaying mathematics and ignore semantic value 2.1. They are usually vendor specific making them unpractical for use in mathematical web publishing.