要使用CSS在border-top属性上实现动画，您可以尝试运行以下代码
<!DOCTYPE html>
<html>
<head>
<style>
table,th,td {
border: 1px solid black;
}
#newTable {
width: 500px;
height: 300px;
background: yellow;
border: 15px solid yellow;
animation: myanim 3s infinite;
background-position: bottom left;
background-size: 50px;
}
@keyframes myanim {
30% {
background-color: orange;
border-right-color: red;
border-right-width: 25px;
border-spacing: 50px;
border-top: 125px solid red;
}
}
</style>
</head>
<body>
<h2>Performing Animation for top border</h2>
<table id = "newTable">
<tr>
<th>Subject</th>
<th>Student</th>
<th>Marks</th>
</tr>
<tr>
<td>Maths</td>
<td>Amit</td>
<td>98</td>
</tr>
<tr>
<td>Science</td>
<td>Sachin</td>
<td>99</td>
</tr>
</table>
</body>
</html>