<script type="text/javascript">
var interval = null;
function more() {
document.getElementById('td1').style.width = (parseInt(document.getElementById('td1').style.wid th) + 1) + 'px';
if(parseInt(document.getElementById('td1').style.w idth) > 300) {
clearInterval(interval);
}
}
function ButtonClick() {
interval = setInterval(more, 10);
}
</script>
<table border=1><tr><td style="width:100px" id="td1">текст</td></tr></table><br>
<input type="button" value="Нажми меня!" onclick="ButtonClick()">