Highlight asp.net controls using required field validate with javascript

/////////////////////html

  <asp:TextBox ID="TxtUserName" OnTextChanged="Txtuser_TextChanged" CssClass="form-control" runat="server" AutoPostBack="True"></asp:TextBox>

<asp:RequiredFieldValidator ID="RFVTxtUserName" runat="server" ControlToValidate = "TxtUserName" Display="Dynamic" ValidationGroup="ValReg"/>

//////////JS

<script type="text/javascript">

        function Validate(ValGrp) {

            var FLAG;

            debugger;

            if (Page_Validators.length > 0) {

                for (var i = 0; i < Page_Validators.length; i++) {

                    var val = Page_Validators[i];

                    var ctrl = document.getElementById(val.controltovalidate); //Validate                   

                    if (ctrl != null && ctrl.style != null) {

                        if (ctrl.value == "" && (ctrl.getAttribute('style') != 'display: none;') && (ctrl.getAttribute('style') != 'border: 1px solid red; display: none;')) {

                            if (Page_Validators[i].validationGroup == ValGrp ) {

                                ctrl.style = "border: 1px solid red";

                                FLAG = 1;

                            }

                        }

                        else {

                            ctrl.style.borderColor = '';

                        }

                    }

                }

            }

            if (FLAG == 1) {

                return false;

            }

            else {

                return true;

            }

        }

        </Script>