<details> element does not show triangle in Firefox

<details> element does not show a triangle in Firefox (Click me to collapse/fold in the screenshots below):

Chrome	Firefox

(https://docs.gitlab.com/ce/user/markdown.html#details-and-summary)

The solution seems to be display: list-item for the <summary> element:

Fully standards-compliant implementations automatically apply the CSS display: list-item to the <summary> element.

(https://developer.mozilla.org/en-US/docs/Web/HTML/Element/details)