Before:

After:

Demo

Firefox Greasemonkey and Safari GreaseKit: just click on nice-alert.user.js

Opera: download nice_alert.user.js and rename it to nice-alert.js (without “user”). Put it into your UserJS directory (can be found at opera:about)

Google Chrome extension

The information-theoretic efficiency E of an interface is defined as the minimum amount of information necessary to do a task, divided by the amount of information that has to be supplied by the user. As is true of physical efficiency, E is at least 0 and is at most 1. Where no work is required for a task and no work is done, the efficiency is defined as 1.

E can be 0 when the user is required to provide information that is totally unnecessary. Surprisingly, a number of interface details achieve the dubious honor of having E=0. A dialog box that allows the user only one possible action, such as clicking the box’s OK button, is such an example. (JavaScript has a command, Alert, solely for creating such unnecessary boxes: The designers were wise enough to remove goto from the JavaScript language to force structured code, but they failed to provide similar guidance on the interface side.)

— from “The Human Interface” by Jef Raskin, pages 84—85.