A modal dialog around a native <dialog> that you write: Alder's renderer writes it for you (heading, close button and all), other stacks write it by hand, as here. Opened by commandfor / command="show-modal" on a button. The no-JS form comes first.
A link to the dialog's id shows it (:target); the close link # hides it. No script, no invoker commands.
The shape Alder emits for dismissible: the close form first, the heading <h2 slot="header"> with an id the dialog's aria-labelledby names, then a slot="description" line in muted text.