The ones I've seen either render LaTeX math using something like MathJax or send your code to a LaTeX process on a server and then display the results as a PDF.

The ones I've seen either render LaTeX math using something like MathJax or send your code to a LaTeX process on a server and then display the results as a PDF. The dumb but tempting bit is using LaTeX to render the entire page without the browser PDF viewer. You can export LaTeX to HTML, for example, but that gives up control of a lot of rendering, including kerning of non-math text.