How can I build an accordion without Javascript using the Field Group module?
Yes, you can use the HTML 5 details tag.
On the content type display administration page, create a field group.
For the Format, chose HTML element.
For Field group label, enter the text you want to show to click to display the rest of the text.
For the Element, enter details.
For Show Label, select Yes.
For Label element, enter summary.
To style with CSS, add extra CSS classes as needed.