Remove `dialog` from the HTML5 shim style (#26269)

Remove `dialog` from the HTML5 shim style (#26269)

`<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.