An unblockable popup uses Dynamic HTML and content layers to avoid
detection by popup blocker programs.
A traditional windowed popup
exists outside the user’s usual web browsing experience.
Therefore, a
popup blocker can detect the webpage, issue a command to open a new
window, and block the popup.
However, an unblockable popup uses
moveable elements of HTML called layers to appear over the normal web
content, but not scroll with it.
As these contents are integral pieces
of the web browsing experience, they are not usually detectable by such
software.
Seasoned users may be familiar with terminate and stay resident
programs, which were popular with consumers of DOS software.
These
programs would activate when a certain hotkey was pressed, and float
right over whatever text the person was looking at.
Unblockable popup
technology works in much the same fashion, and since an unblockable popup layers over the usual content, it usually is not taken to be as
annoying as a windowed popup by most users.
This also can be seen as a great way to increase branding and trust in a website.