The Opera browser has a unique feature in that it allows full-page zoom, where you zoom both images and text, as opposed to most other browsers' text size feature.
Users of the Opera web browser should be seeing a zoom indicator in the upper left corner of the screen that displays the current zoom level, and it should remain roughly the same size as you resize the page. The presented result is fairly accurate when the zoom levels are not to extreme, but as you zoom in, the precision is ultimately limited by two factors: Window size and float precision. With a browser window running full-screen on 640×480, the first erronous result appears when zoom level is 500%, when the calculation returns 520%, at 1024×768, the first error appears at 660%, where the calculation returns 650%.
This demonstration is attached to the weblog entry How to detect zoom in Opera.