Pop-up ad

Users of websites and web applications continuously experience unwanted pop-up ads throughout the course of their normal interaction with a web browser.[7] 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).[5]

A hover ad or in-page pop-up uses JavaScript to combine a banner ad and a pop-up window that appears in front of the browser screen. JavaScript imposes an advertisement over a webpage in a transparent layer. This advertisement can appear in a variety of forms. For example, an advertisement can contain an Adobe Flash animation that links to the advertiser's site. An advertisement can also look like a normal window. Pop-up blockers cannot block the ad because it is a part of the webpage, but it can be blocked with third-party ad blockers such as AdBlock and Adblock Plus, or by using custom style sheets.

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[8] 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.)[9] 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.[10]
• 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.[11]
• 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.

A pop-under ad involves two JavaScript functions introduced in 1995 with the Netscape 2.0b3 browser. Modern web publishers and advertisers use it to create a window in front of the user's screen, load an advertisement, and then send it behind the screen.

// 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.

Hover ads, more commonly called in-page pop-ups, are a special type of pop-up ads created using Dynamic HTML, JavaScript and similar web browser technologies. Because they do not scroll with the web page, they appear to "hover" over the page, usually obscuring the content. Hover ads tend to be very hard to block by pop-up blocking software because the hover ad window is an integral part of the HTML content of the web page. Thus software filtering the content has no algorithmic means of recognizing and removing parts of the content, either descriptive or procedural, that create, populate and manipulate the hover ad's window.