导航
1 后代选择器: div span {color: cadetblue;}
测试2 div->span->p
测试3 div->p->span
2 子代选择器: father > son {color: lightblue;}
3 并集选择器: item1,item2,item3 {color: powderblue;}
4 交集选择器: span.color {color: powderblue;}