其实就是事先在网页里加入两个div框，控制显隐实现的功能。
以下为实现代码（非本人原创，网上找的，我对CSS不感冒）：
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <title>DIV CSS遮罩层</title> <script language="javascript" type="text/javascript"> function showDiv() { document.getElementById("bg").style.display ="block";//遮罩层 document.getElementById("show").style.display ="block";//弹出层 } function hideDiv() { document.getElementById("bg").style.display =‘none‘; document.getElementById("show").style.display =‘none‘; } </script> <style type="text/css"> #bg{ display: none; position: absolute; top: 0%; left: 0%; width: 100%; height: 100%; background-color: black; z-index:1001; -moz-opacity: 0.7; opacity:.70; filter: alpha(opacity=70);} #show{display: none; position: absolute; top: 25%; left: 22%; width: 53%; height: 49%; padding: 8px; border: 8px solid #E8E9F7; background-color: white; z-index:1002; overflow: auto;} </style> </head> <body> <input id="btnshow" type="button" value="Show" onclick="showDiv();"/> <div id="bg"></div> <div id="show">测试<input id="btnclose" type="button" value="Close" onclick="hideDiv();"/></div> </body> </html>
原文：http://www.cnblogs.com/sukra/p/4469542.html