【求解】为什么onclientclick返回false还执行onclick事件？
OnClientClick="return checkuid()"

<script type="text/javascript">
        function checkUID(x,y) {
           var _result = false;  
                 var _txt1 = document.getElementById("txt_username"); 
                 var _txt2 = document.getElementById("txt_pwd");
                 if (_txt1.value == "" && _txt2.value == "") {  
                  alert("请输入用户名和密码");
              }  else {
              if (_txt1.value != "" && _txt2.value == "")
              { alert("请输入密码"); }
            else { _result = true; } 
               }
              }
    </script>

------解决方案--------------------

<script type="text/javascript">

        function checkUID(x,y) {

                 var _txt1 = document.getElementById("txt_username"); 

                 var _txt2 = document.getElementById("txt_pwd");

                 if (_txt1.value == ""
------解决方案--------------------
 _txt2.value == "") {  

                  alert("请输入用户名和密码");

                  return false;

              } 

              return true;

              }

    </script>