From https://once.com/writebook

lib/markdown_renderer.rb

@@ -0,0 +1,33 @@
+require "rouge/plugins/redcarpet"
+
+class MarkdownRenderer < Redcarpet::Render::HTML
+  include Rouge::Plugins::Redcarpet
+
+  def self.build
+    renderer = MarkdownRenderer.new(ActionText::Markdown::DEFAULT_RENDERER_OPTIONS)
+    Redcarpet::Markdown.new(renderer, ActionText::Markdown::DEFAULT_MARKDOWN_EXTENSIONS)
+  end
+
+  def initialize(*args)
+    super
+    @id_counts = Hash.new(0)
+  end
+
+  def header(text, header_level)
+    unique_id(text).then do |id|
+      "<h#{header_level} id='#{id}'>#{text} <a href='##{id}' class='heading__link' aria-hidden='true'>#</a></h#{header_level}>"
+    end
+  end
+
+  def image(url, title, alt_text)
+    %(<a title="#{title}" data-action="lightbox#open:prevent" data-lightbox-target="image" data-lightbox-url-value="#{url}?disposition=attachment" href="#{url}"><img src="#{url}" alt="#{alt_text}"></a>)
+  end
+
+  private
+    def unique_id(text)
+      text.parameterize.then do |base_id|
+        @id_counts[base_id] += 1
+        @id_counts[base_id] > 1 ? "#{base_id}-#{@id_counts[base_id]}" : base_id
+      end
+    end
+end