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A variation on the pop-up window is the pop-under advertisement, which opens a new browser window hidden under the active window. Pop-unders do not interrupt the user immediately and are not seen until the covering window is closed, making it more difficult to determine which web site opened them.
Opera was the first major browser to incorporate tools to block pop-up ads; the Mozilla browser later improved on this by blocking only pop-ups generated as the page loads. In the early 2000s, all major web browsers except Internet Explorer allowed the user to block unwanted pop-ups almost completely. In 2004, Microsoft released Windows XP SP2, which added pop-up blocking to Internet Explorer.
Most modern browsers come with pop-up blocking tools; third-party tools tend to include other features such as ad filtering.
Circumventing pop-up blockers
Users of websites and web applications continuously experience unwanted pop up ads through the course of their normal interaction with a web browser. Ordinarily users respond by dismissing the pop-up through the "close" or "cancel" feature of the window hosting the pop-up. Because this is a typical response, some authors of pop-up advertising depend on this, and create on-screen buttons or controls that look similar to a "close" or "cancel" option. When the user chooses one of these "simulated cancel" options, however, the button performs an unexpected or unauthorized action (such as opening a new pop-up, or running unauthorized commands on the user's system).
Because the technologies for web development and design allow an author to draw any kind of "simulated" cancel option imaginable, some users refuse to click on or interact with any item inside a pop-up window whatsoever.
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