Con il tuo codice ho provato e mi ferma il browser!(IIS6)
Ti allego l'output generato, così lo puoi provare come file.htm
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" >
<HTML>
	<HEAD>
		<title>prova</title>
	</HEAD>
	<body MS_POSITIONING="GridLayout">
		<form name="Form1" method="post" action="WebForm1.aspx" language="javascript" onsubmit="if (!ValidatorOnSubmit()) return false;" id="Form1">
<input type="hidden" name="__VIEWSTATE" value="dDwtMjEwODMyNjM3NDs7PsWhLk01W7DBzAGanmmxnUUnv40X" />
	
<script language="javascript" type="text/javascript" src="/aspnet_client/system_web/1_1_4322/WebUIValidation.js"></script>


			<input name="Texprova" type="text" id="Texprova" style="Z-INDEX: 101; LEFT: 8px; POSITION: absolute; TOP: 8px" />
			<input type="submit" name="Button1" value="Button" onclick="if (typeof(Page_ClientValidate) == 'function') Page_ClientValidate(); " language="javascript" id="Button1" style="Z-INDEX: 103; LEFT: 120px; POSITION: absolute; TOP: 224px" />
			<span id="RequiredFieldValidator1" controltovalidate="Texprova" errormessage="RequiredFieldValidator" evaluationfunction="RequiredFieldValidatorEvaluateIsValid" initialvalue="" style="color:Red;Z-INDEX:102;LEFT:168px;POSITION:absolute;TOP:112px;visibility:hidden;">RequiredFieldValidator</span>
		
<script language="javascript" type="text/javascript">
<!--
	var Page_Validators =  new Array(document.all["RequiredFieldValidator1"]);
		// -->
</script>

			
<script language="javascript" type="text/javascript">
<!--
var Page_ValidationActive = false;
if (typeof(clientInformation) != "undefined" && clientInformation.appName.indexOf("Explorer") != -1) {
    if (typeof(Page_ValidationVer) == "undefined")
        alert("Impossibile trovare la libreria di script '/aspnet_client/system_web/1_1_4322/WebUIValidation.js'. Provare a inserire il file manualmente oppure reinstallarlo eseguendo 'aspnet_regiis -c'.");
    else if (Page_ValidationVer != "125")
        alert("Questa pagina utilizza una versione di WebUIValidation.js non corretta. La versione prevista è 125. La libreria di script è " + Page_ValidationVer + ".");
    else
        ValidatorOnLoad();
}

function ValidatorOnSubmit() {
    if (Page_ValidationActive) {
        return ValidatorCommonOnSubmit();
    }
    return true;
}
// -->
</script>
        

		</form>
	</body>
</HTML>
Ciao
Legnetto