Mermaid.js is a great tool to make diagrams in plain text, I use it a lot and I wanted to have a way to see previews of the code I was writing. There are some options to do that but they require to have a mermaid-cli installed, which requires nodejs as well.

Emacs has a built-in webkit browser, in case it was compiled with --with-xwidgets flag, and mermaid run on js so it should be possible to just run the code I want in the browser and see it there.

This function makes the magic, it just take a region previously selected, which have the mermaid code, create a temp file and write some HTML there(including our mermaid code).

(defun my/preview-mermaid ()
  "Render region inside a webit embebed browser."
  (unless (region-active-p)
    (user-error "Select a region first"))
  (let* ((path (concat (make-temp-file (temporary-file-directory)) ".html"))
         (mermaid-code (buffer-substring-no-properties (region-beginning) (region-end))))
        (insert "<body>
  <pre class=\"mermaid\">")
        (insert mermaid-code)
        ;; js script copied from mermaid documentation
        (insert "</pre>
  <script type=\"module\">
    import mermaid from '';
    mermaid.initialize({ startOnLoad: true });
        (write-file path)))
    (xwidget-webkit-browse-url (format "file://%s" path))))