I gotta question about the popover spec (https://html.spec.whatwg.org/#flow-content-3). I was noticing that it defines a default user-agent style for [popover] to have a padding of 0.25em.

Why were the em units chosen and not px?

I tend to understand em/rem as being related to accessability, where sometimes users need a larger font size to make things easier to read, however, these users don't want to scale everything up (otherwise they would use the browser zoom feature). Sometimes we also need to have containers holding text use rem instead of px so they can continue to properly contain the text, even if a larger default font-size is used. But popover padding? Why use em for that?

I did try searching around the whatwg github repo a bit for relavent discussions, but I'm not well versed yet in how whatwg's content and discussions are organized.