Some initial feedback:
- I glazed over the new details element when I first encountered it. It just didn’t register as a
[details]. It might be “just because it’s new”, but… - It’s divorced from previous design and the default browser implementation – the chevron is after the summary instead of before, and the closed state points in the direction that previously (and by default) meant the opened state.
closed:

open:

- The button/summary element takes much more space (previously it was a semi-inline element)
vs - As seen above, previously the triangle/chevron was aligned with the text and now the “Summary” text isn’t, because of button’s padding (nothing is aligned with anything
)
- On mobile, after interacting with it, button’s background stays highlighted
- I would have voted for the border rather than the background option – I’d prefer that hiding some content did just that, without affecting contrast (relevant mostly in case of embedded images and emoji)
update:
- A reduced clickable area:
vs






