<style>

#ex1{

    border: thin dotted #FF0000;

}

</style>

<script>

function displayResult(){

    document.getElementById("ex1").style.borderTop="thick solid #0000FF";

}

</script>

<div id="ex1">这是一些文本</div>

<br>

<button type="button" onclick="displayResult()">修改上边框</button>

​