test.html

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<html>
<head>
<style type="text/css">
  Label {
   color: black;
  }
  Label.hlt {
   color: red;
  }
</style>

<script type="text/javascript">

function trim(str){
     return str.replace(/(^/s*)|(/s*$)/g, "");
}

function checkInput() {
   if(trim(document.myForm.MustSelect.value)=="")
   {
  alert("本项为必填项！表单不会提交！请补充！");
  document.myForm.MustSelect.focus();
  document.getElementById('test').className ='hlt';
  return false;
   }else{
  alert("表单将会提交！请按确定！");
  return true;
   }
}
</script>
</head>

<body>
   <form name=myForm action="aaa" method="post" onSubmit="return checkInput()">
  <Label id="test">输入必须填写的项目，空格通过trim视为空：</Label>
  <br/>
  <input name="MustSelect"> </input>
  <br/>
  <input type="submit" value="提交"></input>
   </form>
</body>
</html>