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jekyll-mathjax-csp

Render math on the server using the MathJax 3 node API, 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 MathJax 3 node API and thus behaves exactly as client-side MathJax in SVG rendering mode would.

Usage

  1. Install the npm packages mathjax-full and yargs from your top-level Jekyll directory:

    npm init -f # only if you don't have a package.json yet
    npm install mathjax-full yargs
  2. Install jekyll-mathjax-csp:

    gem install jekyll-mathjax-csp
  3. Ensure that your _config.yml contains the following settings:

    plugins:
      - jekyll-mathjax-csp
    
    exclude:
      - node_modules
      - package.json
      - package-lock.json
  4. 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.

  5. Include beautiful math in your posts!

Dependencies

  • mathjax-full (npm): 3.0+
  • yargs (npm): 16.1.0+
  • html-pipeline: 2.3+
  • jekyll: 3.0+

Configuration

MathJax 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

Other configuration options for MathJax are also available:

Key Description Default
em_size Em-size, in pixels 12
ex_size Ex-size, in pixels 6
single_dollars Allow single-dollar delimiters for inline math false
output Output format: SVG or CommonHTML SVG

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

MIT

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Server-side MathJax rendering for Jekyll with a strict CSP

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