JS在网页非当前状态下获取键盘输入值（感觉好难啊，没什么头绪）
请教各位高手一个问题
现有如下一段代码（网上找的）：
<HTML><HEAD><TITLE>js 按键记录</TITLE><META NAME="Generator" CONTENT="EditPlus"><META NAME="Author" CONTENT="羽殇仁"><META NAME="Keywords" CONTENT="js 按键记录"><META NAME="Description" CONTENT="js 按键 记录"></HEAD><BODY><script type="text/javascript">var keystring = "";//记录按键的字符串function $(s){return document.getElementById(s)?document.getElementById(s):s;}function keypress(e){var currKey=0,CapsLock=0,e=e||event;currKey=e.keyCode||e.which||e.charCode;CapsLock=currKey>=65&&currKey<=90;switch(currKey){ //屏蔽了退格、制表、回车、空格、方向键、删除键 case 8: case 9:case 13:case 32:case 37:case 38:case 39:case 40:case 46:keyName = "";break; default:keyName = String.fromCharCode(currKey); break;}keystring += keyName;}function keydown(e){var e=e||event;var currKey=e.keyCode||e.which||e.charCode;if((currKey>7&&currKey<14)||(currKey>31&&currKey<47)){switch(currKey) { case 8: keyName = "[退格]"; break; case 9: keyName = "[制表]"; break; case 13:keyName = "[回车]"; break; case 32:keyName = "[空格]"; break; case 33:keyName = "[PageUp]"; break; case 34:keyName = "[PageDown]"; break; case 35:keyName = "[End]"; break; case 36:keyName = "[Home]"; break; case 37:keyName = "[方向键左]"; break; case 38:keyName = "[方向键上]"; break; case 39:keyName = "[方向键右]"; break; case 40:keyName = "[方向键下]"; break; case 46:keyName = "[删除]"; break; default:keyName = ""; break;}keystring += keyName;}$("content").innerHTML=keystring;}function keyup(e){$("content").innerHTML=keystring;}document.onkeypress=keypress;document.onkeydown =keydown;document.onkeyup =keyup;</script><input type="text" /><input type="button" value="清空记录" onclick="$('content').innerHTML = '';keystring = '';"/><br/>请按下任意键查看键盘响应键值：<span id="content"></span></BODY></HTML>