The CSS overlay property was added as an opt-in to keep top layer elements in the top layer during their exit animation. Feedback from authors using this feature has repeatedly shown that it is easy to miss and that it creates bugs that are hard to track down. Instead of requiring authors to include the overlay property in their CSS transitions, it would be much better to just keep these elements in the top layer automatically during their exit animations. This would fix bugs in existing sites using top layer animations if they forgot to include the overlay property, and it would not affect sites already using the overlay property because including an invalid property in a list of transitions does not break other transitioning properties. The overlay property has not been implemented in other browsers, and it has been rejected for inclusion in WPT interop multiple times: - https://github.com/mozilla/standards-positions/issues/777 - https://github.com/WebKit/standards-positions/issues/169 - https://github.com/web-platform-tests/interop/issues/581 - https://github.com/web-platform-tests/interop/issues/789 - https://github.com/web-platform-tests/interop/issues/1031
Feedback from authors using this feature has repeatedly shown that it is easy to miss and that it creates bugs that are hard to track down. Instead of requiring authors to include the overlay property in their CSS transitions, it would be much better to just keep these elements in the top layer automatically during their exit animations.