If so, it appears that rendering on GitHub.com has display:inline-block for <summary> tags and looks like this:

And on GitHub Pages + Jekyll with markdown: GFM processor it turns out as

Adding
markdown: CommonMarkGhPages
commonmark:
options: ["UNSAFE", "SMART", "FOOTNOTES"]
does not help
The code is:
<details>
<summary>
# Header
</summary>
Para*graph*
</details>
How can I render this in GitHub Pages to be identical to github.com?
@anticomputer Or should this issue be transferred to https://github.com/github/cmark-gfm/issues , if it's more relevant there?
Thanks :)