Declaratively open popovers (and other elements?) on page load

[josepharhar]

What is the issue with the HTML Standard?

In the original proposal for the popover attribute, we had a defaultopen attribute which would show the popover declaratively on page load.

We considered renaming it to popoverdefaultopen for popovers, but then decided not to and instead add the defaultopen attribute in a way that can also be supported on dialog and details elements, potentially blocking on removing the open attribute from them: #7785 (comment)

It doesn't seem like we fully came to an agreement on a plan for defaultopen in that thread, but I've gotten feedback that this would be a useful feature and that we should still try to add it.

@mfreed7 @annevk @domenic