<html><body onmousedown="isKeyPressed(event)"><p>单击文档中的某处。当 onmousedown 事件发生时，警告框会提示您 CTRL 键是否被按下。</p><p><b>提示：</b>请在单击文档之前尝试按住 CTRL 键（在您的键盘上）。</p><script>function isKeyPressed(event) { if (event.ctrlKey) { alert("The CTRL key was pressed!"); } else { alert("The CTRL key was NOT pressed!"); }}</script></body></html>