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A variation on the pop-up window, the pop-under advertisement, opens a new browser window under the active window. Pop-unders do not interrupt the user immediately but appear when the user closes the covering window, making it more challenging to determine which website created them.
Web development and design technologies allow an author to associate any item on a pop-up with any action, including with a cancel or innocent-looking button. Because of bad experiences and apprehensive of possible damage that they may cause, some users do not click on or interact with any item inside a pop-up window whatsoever, and may leave the site that generated them or block all pop-ups.
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 let users block unwanted pop-ups almost completely. In 2004, Microsoft released Windows XP SP2, which added pop-up blocking to Internet Explorer. Most modern browsers provide pop-up blocking tools; third-party tools add other features, such as ad filtering.
Users of websites and web applications continuously experience unwanted pop-up ads throughout 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, the button performs an unexpected or unauthorized action (such as opening a new pop-up or downloading an unwanted file on the user's system).
Circumventing pop-up blockers
URLs are sometimes redirected to advertisement pages by URL redirection.
URLs are sometimes opened in a new tab and then the content of the old background tab will be replaced with an advertisement page by URL redirection, other times it switches the tab the user is on to the advertisement tab. Adblock Plus and NoScript cannot block these redirects.
- Certain types of downloaded content, such as images, free music, and others, can cause pop-ups, and therefore should not be trusted, especially pornographic sites' pop-ups (known as a "pornado" or "porn-storm", as coined by John C. Dvorak.) Also, the pop-ups sometimes look like ordinary web pages, and the name of the site shows up in a search bar.
- Many websites use pop-ups to display information without disrupting the page currently open. For example, they may provide needed extra guidance when filling in a form on a web page, without causing the loss of any information already entered into the form. Most pop-up blockers allow this kind of pop-up. Some, however, reload the page, losing any information the user entered.
- Some online shops use exit intent popups, for example offering discounts, in an attempt to retain users.
- Some web based installers, such as that used by McAfee, use a pop-up to install software.
- On many internet browsers, holding down the ctrl key while clicking a link makes it bypass the popup filter.
- Clicking (even accidentally) on one pop-up may open other pop-up ads.
Pop-under ads are similar to pop-up ads, but the ad window appears hidden behind the main browser window rather than superimposed in front of it. As pop-up ads became more widespread and intrusive, often taking up the whole computer screen, many users would immediately close the pop-up ads that appeared over a site without looking at them. Pop-under ads do not immediately impede the view of content, but remain unnoticed until the user closes or minimizes the main browser window.
Pop-under ad technology
// create a new window in front of the current site window.open(URL, windowName[, windowFeatures]); // push the loaded advertisement back behind the browser window.focus();
Most modern browsers allow
window.open to execute only if it was called by a user interaction (e.g., a mouse click) event handler. Any non-interactive calls (timer callback, load events, etc.) to
window.open result in the new window being blocked.
To bypass this restriction, most pop-under ads trigger on a mouse click event listener attached directly to the document or the document's body. This enables catching all mouse click events that were not consumed by other click event handlers, and calling
window.open without being blocked. For example, when the user selects a text, the mouse click triggers the mouse click handler attached to the document and a pop-under opens using the above code. Other techniques to bypass the
window.open call restriction do so by "hijacking" mouse clicks.
Most pop-up advertising tools include built-in measurement possibilities. These are often dashboards that offer a detailed analysis of your current and previous pop-up ads. Typically, measurable items include:
- Conversion rate or how pop-ups impact the quantity of targeted website activity.
- Number of website visitors. Pop-ups that are inappropriate could hurt the visitors to your website. Counting visitors will enable you to keep track of the promotions, pages, and content that consumers enjoy the most, as well as the devices they use to access your website.
- Drop-off rate. This measure aids in locating any particular pop-up fields that cause website visitors to leave before they can read your content.
- Timing. Understanding if you are delivering the pop-ups in the right time affects the number of visitors and impressions you are getting from them and adjusting display rules accordingly. 
Copyright aspects of pop-up advertising are discussed in the Wikipedia articles on derivative works and transformativeness. Both articles contain illustrations and links to examples of pop-up advertising.
- 1-800 Contacts, Inc. v. WhenU.com, Inc., lawsuit between 1-800 Contacts and WhenU SaveNow over pop-up ad placement
- Ad serving
- Adware and Spyware
- Dark pattern
- Direct marketing
- In-session phishing
- Interstitial webpage
- List of pop-up blocking software
- Messaging spam
- Modal window
- Technical Support Scam
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