<html><body><iframe src="https://www.cnn.com"></iframe><iframe src="https://www.bbc.com"></iframe><iframe src="https://www.nytimes.com"></iframe><p>单击该按钮可获取在此页面上找到的帧数。</p><button onclick="myFunction()">试一试</button><p id="demo"></p><script>function myFunction() { var x = window.length; document.getElementById("demo").innerHTML = x;}</script></body></html>