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<!DOCTYPE html> <html> <head> <style> .test { background-color: yellow; } </style> </head> <body style="height:1500px"> <p>向下滚动此页面</p> <p id="myP" style="position:fixed">当您从该页面顶部滚动 50 像素时，将类“test”（黄色背景色）添加到该段落。 再次向上滚动以删除该类。 </p> <script> window.onscroll = function() {myFunction()}; function myFunction() { if (document.body.scrollTop > 50 || document.documentElement.scrollTop > 50) { document.getElementById("myP").className = "test"; } else { document.getElementById("myP").className = ""; } } </script> </body> </html>