GitHub spoiler syntax

[AustenLamacraft]

I'd like to display GitHub spoiler syntax correctly

Spoiler warning

It doesn't work

<details>
  <summary>Spoiler warning</summary>
  
  It doesn't work
  
</details>

I've tried the gfm extension with no joy. Any ideas?

[ChristianMurphy]

The content shown is HTML, not CommonMark or GFM per se.
To enable HTML, you may want to look into the allowdangeroushtml option (though be aware of the security implications)