[jraph]

The best feature for me is that it works well without JavaScript. The document is still a document.

Except for collapsed code blocks. I would suggest collapsing them using JavaScript, so they are fully readable without. They are currently unavailable to someone who does not run JavaScript.

[deadbyte]

<details> is a native HTML accordion

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You can also add an open attribute to specify that you want the <details> element opened by default