TeX and its most popular utility LaTeX, it now not use only in mathematics but in, Computer Science, Physics, Engineering , Chemistry, Linguistics, and other academic fields. Doing Math well will give other good side effects, such as Greater control over layout, and easy multi-Lingual Documents. As well as being a great technical challenge. Security and Copyright. Many Standard Libraries, and utilities.
Browsers have extensions.
The big different to note here is that the font are really use chosen so the write has not control of the font chose which is very bad for writing mathematics. Currently most common way, files are placed as links in HTML pages. There are hypertext DVI previewers out there. Haven't tried any. Requires action on the user's part.