As others have pointed out, this has nothing to do with with markdown or github. The details and summary elements are part of HTML5. They've been implemented

As others have pointed out, this has nothing to do with with markdown or github. The details and summary elements are part of HTML5. They've been implemented in WebKit and Gecko since at least 2016. https://html.spec.whatwg.org/multipage/interactive-elements.html#the-details-element https://html.spec.whatwg.org/multipage/interactive-elements.... https://developer.mozilla.org/en-US/docs/Web/HTML/Element/details https://developer.mozilla.org/en-US/docs/Web/HTML/Element/de... The more you know!