這里ul的class定義了嗎？
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2017-05-08
最前面引入了外部的css
.clearfix:before, .clearfix:after {
? ?content:"";
? ?display:table;
}
.clearfix:after{
clear:both;
overflow:hidden;
}
.clearfix{
? ?zoom:1;
}
2017-03-08
該位置的class是定義的，多個class之間使用空格進行分割；
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這么寫，相當于選擇器.clearfix .unstyled .drop-menu 三個設置的樣式都會對該元素ul生效