<html><head><style>#example1 { border: 2px solid black; padding: 25px; background: url(mountain.jpg); background-repeat: no-repeat; background-size: contain;}</style></head><body><h2>background-size: contain:</h2><div id="example1"> <h2>Hello World</h2> <p>调整背景图像的大小以确保图像完全可见。</p></div></body></html>