One blank line fixes your details block

There is no Markdown syntax for it

No flavour has one. Not CommonMark, not GitHub Flavored Markdown. Every collapsible section is really the HTML details and summary pair.

Without the blank line, nothing parses

The parser opens an HTML block and copies everything through untouched. Your bold stays two asterisks. Your bullet list stays hyphens.

With it, everything renders

An empty line ends the HTML block, so Markdown is parsed again. Verified with markdown-it 14.3.2: real strong tags, real list items, real tables.

Bold in the summary never works

The summary line sits inside the HTML block, so Markdown is never parsed there. Asterisks show literally. Use a b tag inside the summary instead.

Sanitizers flatten it silently

Neither tag is in sanitize-html's default allow list. The tags vanish, the content stays. Whatever you meant to hide is published in full.

Three patterns worth copying

Add the open attribute to start expanded. Nest two levels, each with its own blank line. Put a whole comparison table inside. All three confirmed working.

On a devbio bio field it prints

The About block renders bold, italic and links only. Raw HTML is escaped by design, so a pasted details tag shows as visible text to every visitor.

The whole rule, in one line

Blank line after the closing summary tag. Everything else follows from it.