<html><head><style>#myDiv { border: 1px solid #FF0000;}</style></head><body><div id="myDiv" style="margin-top:5cm;">这是一个 div。</div><br><button type="button" onclick="myFunction()">返回上边距</button><script>function myFunction() { alert(document.getElementById("myDiv").style.marginTop);}</script></body></html>