获得容器里的被选择文本内容（兼容firefox,opera,ie）

<script type="text/javascript">
<!--
function getFieldSelection(select_field)
{
	word='';
	if (document.selection) {
		var sel = document.selection.createRange();
		if (sel.text.length > 0) {
			word = sel.text;
		}
	}
	else if (select_field.selectionStart || select_field.selectionStart == '0') {
		var startP = select_field.selectionStart;
		var endP = select_field.selectionEnd;
		if (startP != endP) {
			word = select_field.value.substring(startP, endP);
		}
	}
	return word;
}
//-->
</script>
<textarea id="t_field" rows="3" cols="20"></textarea><br />
<span onmousedown="alert(getFieldSelection(document.getElementById('t_field')))" style="color:#6633FF;cursor:pointer">(Span_onmousedown)</span>
<span onclick="alert(getFieldSelection(document.getElementById('t_field')))" style="color:#6633FF;cursor:pointer">(Span_onclick)</span>
<button onclick="alert(getFieldSelection(document.getElementById('t_field')))">button_click</button>