[chrismorgan]

But IE and EdgeHTML-based Edge never supported <details>, so you had to polyfill it somewhat imperfectly. So one of the major browsers only gained support for it in 2020.

This was a significant dissuader, especially when you consider how you can’t transition the height (very often desirable) without added JavaScript. So <details> has really only been seeing much use in the last few years.

(The minimum possible polyfill is just under 750 bytes, supporting IE9+/Edge, requiring <summary> to be provided, and disallowing text children of <details>. The typical polyfill was more like 2–3KB.)