Validate Generated HTML from Markdown

Posted by Misagh Moayyed on April 10, 2020 · 6 mins read

If you have a set of Markdown documents hosted somewhere (i.e. GitHub Pages) and rendered in HTML format, it would be a good idea to validate and proof-read them all, to make sure image references, links, tags, etc are all working correctly before you publish. Doing so manually would take a lot of time and is quite a tedious task. So in this blog post, we are going to take a look at the HTMLProofer tool to see how it can be used to validate our markdown documents.


  • (Optional) Ruby
  • Docker
  • Text Editor (Visual Studio Code, Sublime Text, etc)


First, the documentation instructs us to put together a Ruby script that would mainly depend on the html-proofer Ruby gem to process our documents. That script, html-proofer-docs.rb would be as follows:

require 'html-proofer'
require 'html/pipeline'
require 'find'
require 'fileutils'

# make an out dir
Dir.mkdir("out") unless File.exist?("out")

pipeline = [
], :gfm => true

# iterate over files, and generate HTML from Markdown
Find.find("./docs") do |path|
  if File.extname(path) == ".md"
    contents =
    result =
    dirname = File.dirname(path)
    FileUtils.mkdir_p ("out/" + dirname)
    content_str = result[:output].to_s
    filename = path.split("/").pop.sub('.md', '.html')
    if filename == "sidebar.html"
      content_str = content_str.gsub! '/%24version' '.'
    end"out/#{dirname}/#{filename}", 'w') { |file| file.write(content_str) }
# url_ignore - ignore links content not in branch
# file_ignore - ignore CAS spec b/c it has lots of bad anchor links, only *.html files are processed
options = {
            :file_ignore =>  [ %r{.*/CAS-Protocol-Specification.html} ],
            :disable_external => true,
            :only_4xx => true,
            :empty_alt_ignore => true,
            :url_ignore => [ %r{^/cas}, %r{^../images/}, %r{^../../developer/} ],
# test your out dir!
HTMLProofer.check_directory("./out", options).run

Then, all that is left is to run the script. If you have Ruby installed, you should be able to execute the script directly. Alternatively, I am going to use a Docker image that my colleague, Hal Deadman on the Apereo CAS project, put together to run the script all via a file:

docker run --name="html-proofer" --rm --workdir /root \
    -v $(pwd)/_posts:/root/docs \
    -v $(pwd)/out:/root/out \
    -v $(pwd)/html-proofer-docs.rb:/root/html-proofer-docs.rb \
    --entrypoint /usr/local/bin/ruby \
    hdeadman/html-proofer:latest /root/html-proofer-docs.rb
# rm -Rf ./out

The script above assumes that Markdown documents are found in the current directory inside a _posts folder which is then mapped onto the running container. HTML files will be generated in an out directory and of course, the location of the Ruby script is assumed to also be in the same directory.

If you run the script, hopefully you will see the following output:

Running ["ImageCheck", "ScriptCheck", "LinkCheck"] on ["./out"] on *.html...
Ran on 123 files!
HTML-Proofer finished successfully.

Or, you might see errors:

  *  ( is an invalid URL (line 8)
     <a href="(">GitHub Pages</a>

If your Markdown documents are technical documentation and manuals, let’s say for a project that is hosted on GitHub, you could very easily integrate the above steps into your CI workflow and automate the validation process for all commits and pull requests.


I hope this review was of some help to you. Be sure to take a look at HTMLProofer and give it a try. Of course, kudos to Hal Deadman for putting together the script, the Docker image and the follow-up work that went into the Apereo CAS project to make sure the documentation syntax is kept up to par.

Misagh Moayyed