<dialog>, in browsers that support it, has user agent styles of dialog { display:block; ...} dialog:not([open]) { display:none; }
by forcing it to display:block in the shim, the dialog is shown even when closed. There's no clean way to shim this for non-supporting browsers,
but arguably these browsers would have further problems with pure <dialog> usage anyway, and it's up to authors to then use different elements/shims.