<script type="text/javascript">
	// Firefox, Google Chrome, Opera, Safari, Internet Explorer from version 9
	function OnInput (event) {
		//alert ("The new content: " + event.target.value);
		UpdateSaveButton(event);
	}
	// Internet Explorer
	function OnPropChanged (event) {
		if (event.propertyName.toLowerCase () == "value") {
			//alert ("The new content: " + event.srcElement.value);
		UpdateSaveButton(event);
		}
	}
	function UpdateSaveButton(event) {
		var bIsFilled = false;
		bIsFilled = document.getElementById("tbTest").value.length > 0 && document.getElementById("tbTestQuestion").value.length > 0 && document.getElementById("tbTestAnswer").value.length > 0;
		document.getElementById("btnSaveTest").disabled = !bIsFilled;
	}
</script>

<textarea id="tbTest" runat="server" oninput="OnInput (event)" class="mframe" onpropertychange="OnPropChanged (event)"/>
<textarea id="tbTestQuestion" runat="server" oninput="OnInput (event)" class="mframe" onpropertychange="OnPropChanged (event)"/>
<textarea id="tbTestAnswer" runat="server" oninput="OnInput (event)" class="mframe" onpropertychange="OnPropChanged (event)"/>
<asp:Button id="btnSaveTest" runat="server" Text="Button" Enabled="false"/>