why is <dialog> defaulted to `display: block`?

See this line https://github.com/twbs/bootstrap/blob/8f7bd419935adfcd53c471a0202083464800619e/dist/css/bootstrap-reboot.css#L27

It seems that this is causing the contents of the `<dialog>...</dialog>` to be rendered when the page is loaded.  If bootstrap is not used, it will hide the contents until `myDialog.showModal()` is called.