Document.ActiveElement

<script>
function document.onmousedown()
{
var srcElem = document.activeElement
if (srcElem.value == "") srcElem.onblur = new Function("this.focus()")
else srcElem.onblur = new Function("")
}

function document.onkeydown()
{
if (event.keyCode == 9 && document.activeElement.value == "") return false
else event.srcElement.onblur = new Function("")
}
</script>

<input type="text">
<input type="text">