Overview
User Agent Impersonation is altering the configutation of the browser in order to change the browsers fingerprint so that the new fingerprint matches another browser.
Video Tutorials
Exploitation
To get started impersonating a user-agent, try a tool like User-Agent Switcher; a free add-on for Firefox
Some of the configurations have to be changed by changing the browsers configuration. In Firefox, see about:config. Use Google to determine how to change the configuration values.
The validator on this page is itself a JavaScript. The validator source code can be read to determine what values are required. Also the javascript could be manipulated or even rewritten to beat the challenge. There is no security in javascript.
Videos
Introduction to User-agent Impersonation