Postprocessing HTML in jekyll - jekyll

Question

Is it possible to add a post-processing step (in ruby) to run in Jekyll after it converts markup to HTML?
I'd like to add some html content, and can't see a way to do that in Jekyll files in general (though certain dialects of markup might support it), so I think it would have to be done by operating on the HTML after Jekyll converts it and before it writes it into _site/.
EDIT: Clarified that I'm looking to do this in Ruby and in arbitrary dialects of markup.

Answer 1

It looks like I may be able to do this by providing a Liquid filter that postprocess the html content, and changing {{ content }} to {{ content | my_postprocess }} in _layouts/post.html and _layouts/page.html.

Answer 2

Indeed, kramdown will not parse markdown in html element by default.
But, there is some configuration parameters that can be set to force kramdown to parse markdown in span or block elements.
Kramdown parameters in Jekyll documentation (look under the kramdown: key) but more interesting things in the kramdown documentation particularly here and here
In configuration
If you want to globally parse markdown in html, in _config.yml, add :
kramdown:
parse_block_html: true
parse_span_html: true
Or, in your markdown itself
{::options parse_block_html="true" /}
{::options parse_span_html="true" /}
<div>
## Some markdown here
**bold** and `code`
<cite>a **span** level element</cite>
</div>
You can also use markdown includes like this :
{% capture md %}{% include markdown_file.md %}{% endcapture %}
{{ md | markdownify }}
This will render any markdown as if it was in the original post/page.

Answer 3

Newer versions of Jekyll let you use hooks to do post-processing (and many other things).
For example, you could put a file like this in the _plugins/ directory, and it will modify the contents of posts after they've been converted to HTML but before they've been embedded in a layout file or written to disk:
Jekyll::Hooks.register :posts, :post_convert do |post|
post.content = post.content.gsub('old', 'new')
end