Most GUI environments have a magnifier tool. A magnifier tool is exactly what it means. It can be positioned over a portion of the screen, and magnify the content.
A magnifier is useful for people who need some help to read details.
Older versions of browsers can magnify only by increasing the font size. However, bitmaps retain their sizes and resolutions. That makes the magnification features not very useful for any document that contains graphical elements.
New browsers, such as Firefox version 3+, magnify both font sizes and also bitmaps. This means that it has a built-in magnification tool that can take the place of most magnifiers. The only advantage of an independent magnifier is that it scrolls through different parts of an HTML document like a real magnifier, whereas the built-in zoom feature of a browser reformats an HTML document and wrap text differently.