What is the issue with the HTML Standard?

Currently showModal() and show() on a dialog, assert that the dialog isn't in document's open dialogs list.

However, it turns out this isn't always true.

For example if you do:
dialog.showModal(); // Adds the dialog to the open dialogs list
dialog.removeAttribute('open'); // Dialog is now in a strange state
dialog.showModal(); // Assertion fails because the dialog is already in the open dialogs list

The spec needs attribute changed steps for dialog open attribute being removed and needs to remove the dialog from the list.