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Important: This repo and gem are unmaintained! If you are interested in maintaining, please contact me at

kramdown math engine for conversion to HTML

This is a converter for kramdown that uses KaTeX and the sskatex gem to convert math formulas to HTML+MathML on the server side.

Note: Until kramdown version 2.0.0 this math engine was part of the kramdown distribution.


gem install kramdown-math-sskatex


require 'kramdown'
require 'kramdown-math-sskatex', math_engine: :sskatex).to_html


This math engine uses a server-side installation of KaTeX to convert TeX math formulas into HTML+MathML. This eliminates the need for client-side math-rendering Javascript. Consider this a flexible for-trusted-users-only alternative to the KaTeX math engine and a lightweight efficient alternative to Mathjax-Node.

Your HTML templates need no longer include Javascripts for Math (neither katex.js nor any search-and-replace script). Your HTML templates should continue referencing the KaTeX CSS. If you host your own copy of the CSS, also keep hosting the fonts.

Requirements for running kramdown with math engine SsKaTeX:

All these requirements need to be met only if SsKaTeX is actually used.

A typical SsKaTeX configuration looks like this:

math_engine: sskatex
  katex_js: 'path-to-katex/katex.min.js'

The complete list of available options in math_engine_opts follows. Most options, with the notable exception of katex_opts, do not affect usage nor output, but may be needed to make SsKaTeX work with all the external parts (JS engine and KaTeX). Admins should read the security notes.

katex_js : The path to your copy of katex.min.js. Defaults to 'katex/katex.min.js'. For a relative path, the starting point is the current working directory.

katex_opts : A dictionary with general KaTeX options such as throwOnError, errorColor, colorIsTextColor, and macros. See the KaTeX documentation for details. Use throwOnError: false if you want parse errors highlighted in the HTML output rather than raised as exceptions when compiling. Note that displayMode is computed dynamically and should not be specified here.

js_run : An identifier for the Javascript engine to use. Recognized identifiers include: RubyRacer, RubyRhino, Duktape, MiniRacer, Node, JavaScriptCore, Spidermonkey, JScript, V8, and Disabled; that last one would raise an error on first use. Which engines are actually available depends on your installation.

If js_run is not defined, the contents of the environment variable EXECJS_RUNTIME will be considered instead; and if that is not defined, an automatic choice will be made. For more information, use the verbose option and consult the execjs documentation.

js_dir : The path to a directory with Javascript helper files. Defaults to the subdirectory js in the data directory of the sskatex gem. There is no need to change that setting unless you want to experiment with Javascript details.

js_libs : A list of UTF-8-encoded Javascript helper files to load. Relative paths are interpreted relative to js_dir. The default setting is

  - escape_nonascii_html.js
  - tex_to_html.js

And there is no need to change that unless you want to experiment with Javascript details.

Files available in the default js_dir are:

escape_nonascii_html.js : defines a function escape_nonascii_html that converts non-ASCII characters to HTML numeric character references. Intended as postprocessing filter.

tex_to_html.js : defines a function tex_to_html(tex, display_mode, katex_opts) that takes a LaTeX math string, a boolean display mode (true for block display, false for inline), and a dict with general KaTeX options, and returns a string with corresponding HTML+MathML output. The implementation is allowed to set katex_opts.displayMode. SsKaTeX applies tex_to_html to each math fragment encountered. The implementation given here uses katex.renderToString and postprocesses the output with escape_nonascii_html.

verbose : Whether to log the engine configuration. Defaults to false. When set to something else, logs (by converter warnings) the identifiers of the available Javascript engines, the selected engine (cf. option js_run), the names of the Javascript files loaded, and the contents of katex_opts. That happens only once per new configuration.

debug : For debugging. Defaults to false. When set to something else, prints (by warn) every information that the verbose option would log, and every Javascript expression used after engine initialization, immediately to stderr.


The options with js in their names can be used to achieve arbitrary Javascript code execution with the privileges of the kramdown process. If kramdown is part of a service that allows file uploads and user-specified math engine selection and options, the service should therefore be sandboxed (which is recommended practice), or the user-specified options filtered, or SsKaTeX kept unavailable e.g. by not installing the sskatex gem. Consider using the KaTeX math engine instead.

Differences to the KaTeX math engine

Both the KaTeX and the SsKaTeX engine operate in similar ways with similar efficiency and produce equivalent output if the underlying katex.min.js versions are the same. Differences are mostly in configuration and usage scenarios. The following table gives an overview.

Usage aspect KaTeX SsKaTeX
Usability easy to use all JS details configurable
Target users untrusted users trusted users
Target usage web services personal pages
Required Ruby gem katex sskatex
KaTeX JS/CSS/fonts included not included
Math language depends on katex gem version katex_js option
KaTeX options in math_engine_opts in katex_opts sub-dict
Default error handling catches errors throws on errors


  • If in doubt, try the KaTeX math engine. It should work out of the box.
  • If you need more control and are trusted (i.e. you are allowed to run arbitrary code), try SsKaTeX. Example uses:
    • Work around issues in the current KaTeX version or in the default JS interpreter
    • Try a more recent KaTeX version when you want it, and not until then
    • Try interfacing with other JS-based math renderers
    • Try another Javascript interpreter. E.g. Duktape is fast, but Spidermonkey may give better error diagnostics and backtraces.
  • Web services processing kramdown input from untrusted users should not make the sskatex gem available, as explained in the Security section.


Clone the git repository and you are good to go. You probably want to install rake so that you can use the provided rake tasks.


MIT - see the COPYING file.


kramdown-math-sskatex uses katex to convert math elements to HTML+MathML on the server side








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