jekyll-mathjax-csp

Render math on the server using MathJax-node, while maintaining a strict Content Security Policy (CSP) without 'unsafe-inline'.

While MathJax is well equipped to render beautiful math in a browser, letting it run in the client has two distinctive disadvantages: It is quite CPU-intensive and crucially relies on inline style attributes and elements. This Jekyll plugin aims to resolve both issues at once by rendering formulas to SVG images on the server, extracting all generated style attributes into a single <style> element in the head of the page and computing a hash over its content that can then be added as a CSP style-src.

The plugin runs the output of Jekyll's markdown parser kramdown through the CLI converter mjpage offered by the npm package mathjax-node-page and thus behaves exactly as client-side MathJax in SVG rendering mode would.

Usage

Install the npm package mathjax-node-page from your top-level Jekyll directory:

npm init -f # only if you don't have a package.json yet
npm install mathjax-node-page@3.X

Install jekyll-mathjax-csp:
```
gem install jekyll-mathjax-csp
```

Ensure that your _config.yml contains the following settings:

plugins:
  - jekyll-mathjax-csp

exclude:
  - node_modules
  - package.json
  - package-lock.json

Add the {% mathjax_csp_sources %} Liquid tag where you want the CSP 'sha256-...' hashes for <style> elements to be emitted. Don't forget to add the YAML front matter (two lines of ---) to such files. If you specify your CSP in a different way, add the style-src sources the plugins prints to the console during build.
Include beautiful math in your posts!

Dependencies

mathjax-node-page (npm): 2.0+
html-pipeline: 2.3+
jekyll: 3.0+

Configuration

The following fields can be set in _config.yml; their default values are given in the sample below.

mathjax_csp:
  linebreaks: false
  single_dollars: false
  format: AsciiMath,TeX,MathML
  font: TeX
  semantics: false
  notexthints: false
  output: SVG
  eqno: none
  ex_size: 6
  width: 100
  extensions: ""
  font_url: "https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.2/fonts/HTML-CSS"
  strip_css: false

'mathjax-node-page' adds a fixed inline stylesheet to every page containing math. If you want to serve this stylesheet as an external .css, you can advise the plugin to strip it from the output by adding the following lines to your _config.yml:

mathjax_csp:
  strip_css: true

Configuration for 'mathjax-node-page' is also available:

Key	Description	Default
`linebreaks`	Perform automatic line-breaking	`false`
`single_dollars`	Allow single-dollar delimiters for inline math	`false`
`format`	Input format(s) to look for	`AsciiMath,TeX,MathML`
`font`	Web font to use in SVG output	`TeX`
`semantics`	For TeX or Asciimath source and MathML output, add input in `<semantics>` tag	`false`
`notexthints`	For TeX input and MathML output, don't add TeX-specific classes	`false`
`output`	Output format: SVG, CommonHTML, or MML	`SVG`
`eqno`	Equation number style (none, AMS, or all)	`none`
`ex_size`	Ex-size, in pixels	`6`
`width`	Width of equation container in `ex`. Used for line-breaking	`100`
`extensions`	Extra MathJax extensions (e.g. `Safe,Tex/noUndefined`)	`""`
`font_url`	URL to use for web fonts	`https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.2/fonts/HTML-CSS`

Local testing

If you want to try out your CSP locally, you can specify headers in your _config.yml:

webrick:
  headers:
    Content-Security-Policy: >-
      default-src 'none'; script-src ...

It is unfortunately not possible to have Liquid tags in _config.yml, so with this approach, you will have to update your CSP manually. Don't forget to restart jekyll for it to pick up the config changes.

Another possibility is using a meta tag with http-equiv, placed in your pages' <head> element:

<meta http-equiv="Content-Security-Policy" content="default-src 'none'; script-src ...">

Note that this cannot be used with frame-ancestors, report-uri, or sandbox.

License