<html><head><style>#myDiv { border: thick solid #FF0000;}</style></head><body><div id="myDiv">这是一个 DIV 元素。</div><br><button type="button" onclick="myFunction()">更改左边框样式</button><script>function myFunction() { document.getElementById("myDiv").style.borderLeftStyle = "dotted";}</script></body></html>