xxxxxxxxxx<html><head><meta name="viewport" content="width=device-width, initial-scale=1.0"></head><body><picture> <source srcset="/statics/images/course/img_smallflower.jpg" media="(max-width: 400px)"> <source srcset="/statics/images/course/img_flowers.jpg"> <img src="/statics/images/course/img_flowers.jpg" alt="Flowers" style="width:auto;"></picture><p>调整浏览器宽度和高度，背景在宽度小于 400px 时将改变。 </p></body></html>