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<!DOCTYPE html> <html> <head> <style> #myDIV { border: 1px solid black; width: 300px; height: 300px; } </style> </head> <body> <h1>使用 JavaScript 更改顶部边框</h1> <p>点击“试一试”按钮更改 DIV 元素的 border-top 属性：</p> <button onclick="myFunction()">试一试</button> <div id="myDIV"> <h1>Hello</h1> </div> <script> function myFunction() { document.getElementById("myDIV").style.borderTop = "3px dashed lightblue"; } </script> </body> </html>