1function selectElementContents(el) {
2	var body = document.body, range, sel;
3	if (document.createRange && window.getSelection) {
4		range = document.createRange();
5		sel = window.getSelection();
6		sel.removeAllRanges();
7		try {
8			range.selectNodeContents(el);
9			sel.addRange(range);
10		} catch (e) {
11			range.selectNode(el);
12			sel.addRange(range);
13		}
14	} else if (body.createTextRange) {
15		range = body.createTextRange();
16		range.moveToElementText(el);
17		range.select();
18	}
19}

1<table id="tableId" border="1">
2	<thead>
3		<tr><th>Heading 1</th><th>Heading 2</th></tr>
4	</thead>
5	<tbody>
6		<tr><td>cell 1</td><td>cell 2</td></tr>
7	</tbody>
8</table>
9
10<input type="button" value="select table" onclick="selectElementContents( document.getElementById('tableId') );">