<details> and <summary> are great, but animating them to slide open instead of "pop" is difficult to implement without JS when the contents are not a fixed heig

<details> and <summary> are great, but animating them to slide open instead of "pop" is difficult to implement without JS when the contents are not a fixed height. Sliding open guides the user that something has changed, improves the UX. I've found that using the `open` attribute in CSS behaves differently across browsers as well. Sometimes it doesn't even work consistently in a single browser implementation.