The pseudo-class you're after is :empty, not :only-child. It matches a widget that has no displayed children, so it's exactly the "is this container empty right now" hook you were looking for on the CSS side. It updates live as children mount and unmount.

The catch: Textual only has the descendant (space) and child (>) combinators. No sibling combinators (+, ~), so you can't write #items:empty ~ #placeholder to flip a separate placeholder widget from pure CSS. That's the wall you hit.

Two ways around it, neither of which needs the extra reactive.

If the placeholder can just be the empty container dressed up, style the container and give it a border title:

def compose(self) -> ComposeResult:
    items = Container(id="items")
    items.border_title = "Nothing here yet"
    yield items

#items:empty {
    border: round $panel;
    align: center middle;
}

border_title renders whether or not the container has children, so it shows through when empty and just gets covered once you mount real children.

If you want a distinct placeholder widget, don't fight CSS for it. Toggle it in the same place you already add and remove children:

self.query_one("#placeholder").display = not self.query_one("#items").children

One line, at a point you're already touching, no separate state to keep in sync.