Yet another TeXmath-to-HTML gem. This one uses KaTeX and ExecJS.

This is a TeX-to-HTML+MathML+CSS converter class using the Javascript-based
KaTeX, interpreted by one of the Javascript engines supported by ExecJS.
The intended purpose is to eliminate the need for math-rendering JavaScript
in the HTML browser.

Consider this a lightweight alternative to mathjax-node-cli.
Javascript execution context initialization can be done once and then
reused for formula renderings with the same general configuration.
As a result, the performance is reasonable.

Originally added directly to a kramdown fork. However, relying on config
from potentially untrusted sources to find the necessary Javascript files
might open security vulnerabilities. Therefore we chose to make that
feature upgrade an opt-in requiring dedicated admin action, namely the
explicit installation of this gem.

COPYING

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+SsKaTeX - Server-side KaTeX for Ruby
+(not including KaTeX itself)
+MIT License
+Copyright (C) 2017 Christian Cornelssen <ccorn@1tein.de>
+
+Permission is hereby granted, free of charge, to any person obtaining a
+copy of this software and associated documentation files (the
+"Software"), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish,
+distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to
+the following conditions:
+
+The above copyright notice and this permission notice shall be included
+in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
+TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
+SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

README.md

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-# sskatex
-SsKaTeX - Server-side KaTeX for Ruby
+# SsKaTeX
+## Server-side KaTeX for Ruby
+
+This is a TeX-to-HTML+MathML+CSS converter class using the Javascript-based
+[KaTeX], interpreted by one of the Javascript engines supported by [ExecJS].
+The intended purpose is to eliminate the need for math-rendering Javascript
+in the client's HTML browser. Therefore the name: SsKaTeX means *server-side*
+KaTeX.
+
+Javascript execution context initialization can be done once and then reused
+for formula renderings with the same general configuration. As a result, the
+performance is reasonable. Consider this a fast and lightweight alternative to
+[mathjax-node-cli].
+
+Requirements for using SsKaTeX:
+
+* Ruby gem [ExecJS],
+* A Javascript engine supported by ExecJS, e.g. via one of
+    - Ruby gem [therubyracer],
+    - Ruby gem [therubyrhino],
+    - Ruby gem [duktape.rb],
+    - [Node.js],
+* `katex.min.js` from [KaTeX].
+
+Although the converter only needs `katex.min.js`, you may need to serve the
+rest of the KaTeX package, that is, CSS and fonts, as resources to the
+targeted web browsers. The upside is that 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.
+
+Minimal usage example:
+
+    tex_to_html = SsKaTeX.new(katex_js: 'path-to-katex/katex.min.js')
+    # Here you could verify contents of tex_to_html.js_source for security...
+
+    body_html = '<p>By Pythagoras, %s. Furthermore:</p>' %
+      tex_to_html.call('a^2 + b^2 = c^2', false)  # inline math
+    body_html <<                                  # block display
+      tex_to_html.call('\frac{1}{2} + \frac{1}{3} + \frac{1}{6} = 1', true)
+    # etc, etc.
+
+More configuration options are described in the Rdoc. 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).
+Since KaTeX is distributed separately from the SsKaTeX gem, configuration of
+the latter must support the specification of Javascript file locations. This
+implies that execution of arbitrary Javascript code is possible. Specifically,
+options with `js` in their names should be accepted from trusted sources only.
+Applications using SsKaTeX need to check this.
+
+[duktape.rb]: https://github.com/judofyr/duktape.rb#duktaperb
+[ExecJS]: https://github.com/rails/execjs#execjs
+[KaTeX]: https://khan.github.io/KaTeX/
+[mathjax-node-cli]: https://github.com/mathjax/mathjax-node-cli
+[Node.js]: https://nodejs.org/
+[therubyracer]: https://github.com/cowboyd/therubyracer#therubyracer
+[therubyrhino]: https://github.com/cowboyd/therubyrhino#therubyrhino

Rakefile

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+require 'rake/testtask'
+
+Rake::TestTask.new do |t|
+  t.libs << 'test'
+end
+
+desc "Run tests"
+task default: [:test]

data/sskatex/js/escape_nonascii_html.js

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+// -*- coding: utf-8 -*-
+//
+// Copyright (C) 2017 Christian Cornelssen <ccorn@1tein.de>
+//
+// This file is part of SsKaTeX which is licensed under the MIT.
+
+// Transform non-ASCII characters and '\0' in given string to HTML numeric character references
+function escape_nonascii_html(str) {
+  return str.replace(/[^\x01-\x7F]/g, function (u) {
+    return "&#x" + u.charCodeAt(0).toString(16) + ";";
+  });
+};

data/sskatex/js/tex_to_html.js

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+// -*- coding: utf-8 -*-
+//
+// Copyright (C) 2017 Christian Cornelssen <ccorn@1tein.de>
+//
+// This file is part of SsKaTeX which is licensed under the MIT.
+
+// Given a LaTeX math string, a boolean display_mode
+// (true for block display, false for inline),
+// and a dict katex_opts with general KaTeX options,
+// return a string with corresponding HTML+MathML output.
+// The implementation is allowed to set katex_opts.displayMode .
+function tex_to_html(tex, display_mode, katex_opts) {
+  katex_opts.displayMode = display_mode;
+  return escape_nonascii_html(katex.renderToString(tex, katex_opts));
+};