您可以选择未来选项 t在父级（ DIV.items ）上指定它，但你可以在子级（ DIV.item ） ：only-child 伪类。由于您在这里为子女的后代设计样式，您所要做的事实上是可能的：

You can't specify it on the parent (DIV.items), but you can on the child (DIV.item) with the :only-child pseudo-class. Since you're styling a descendant of the child here, what you're looking to do is in fact possible:

DIV.item:only-child BUTTON.delete {
    display: none;
}