<html><head><style>p#demo { text-decoration: underline;}</style></head><body><p id="demo">Hello world!</p><p>点击“试一试”按钮可改变下划线的颜色。</p><button onclick="myFunction()">试一试</button><script>function myFunction() { document.getElementById("demo").style.textDecorationColor = "red";}</script></body></html>