保证1920px的图片，在低分率率的电脑上也能正常显示，两边裁剪，中间居中，高度不变
<!DOCTYPE html><html lang="en"><head> ?<meta charset="UTF-8"> ?<title>Document</title></head><body><div class="test"> ?<div style="width: 100%;height: 390px; background: url(http://imagecdn0.17huo.com/o_1bvue57hgbp8ms1diu18ctjnrh.jpg) no-repeat center;background-size: 1920px 390px;"></div></div></body></html>