Collapsible header in Markdown to html

Try:

<details>
  <summary>Your header here! (Click to expand)</summary>
  Your content here...
  > markup like blockquote's should even work on github!
  more content here...
</details>

You can try this sort of thing here:

    <details>
      <summary>Your header here! (Click to expand)</summary>
      Your content here...</br>
      (markup only where supported)</br>
      more content here...</br>
    </details>

This works for me with Chrome, but may not work yet for other browsers.
There are some related posts at github.