<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>prompt</title>
<script type="text/javascript">
function rec(){
var score;
score=prompt("你们的考试分数")
if (score>=90)
{
document.write("你很棒！");
}
else if (score>=75)
{
document.write("不错哦！");
else if (score>=60)
{
document.write("要加油！");
}
else
{
document.write("要努力了！");
}
}
</script>
</head>
<body>
<input name="button" type="button" onClick="rec()" value="点击我，对成绩做评价!" />
</body>
</html>
else if (score>=75)
{
document.write("不错哦！");
这里少了个又半大括号吧
score=prompt("你们的考试分数");
你的分号不见了