Dialog
A modal dialog sits on the overlay surface over a blurred backdrop, and the page behind it
doesn't scroll, except in Safari. Under prefers-reduced-transparency, the backdrop darkens
instead of blurring.
Accessibility
Open it with a button's commandfor and command="show-modal", or
showModal() from script, so focus moves into it and Escape closes it. Label it with
aria-labelledby. closedby="any" also closes it on a click outside, where supported;
leave it off a dialog that holds unsaved input.
Customize
It scales in, unless motion is reduced.
Popover
A popover opened by a popovertarget button, with no JavaScript.
A popover opened with popovertarget, no JavaScript needed.
Accessibility
Escape and a click outside close it, and the button tells screen readers whether it's open.
Customize
It slides up into place, unless motion is reduced.