知识点：子集元素 height:0;时 padding-bottom:100% 根据父级元素的宽度来计算。100%比例为1:1 66.66% 比例为 4:3 133% 比例为 3:4
1 .backgroundimg>div{ 2 width: 100%; 3 height: 0; 4 padding-bottom:100%; 5 padding-bottom:66.66%; 6 padding-bottom:133%; 7 overflow:hidden; 8 background-position:center center; 9 background-repeat:no-repeat; 10 background-size:cover!important; 11 -moz-background-size:cover; 12 -webkit-background-size:cover; 13 -webkit-transition:all .6s; 14 -moz-transition:all .6s; 15 -ms-transition:all .6s; 16 -o-transition:all .6s; 17 transition:all .6s; 18 }